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Results
Fluid-Leakoff Delineation in High-Permeability Fracturing
Valko´, P.P. (Texas A&M U.) | Economides, M.J. (Texas A&M U.)
Summary Starting from the original concept proposed by Carter, Howard and Fast, this paper reviews the description of fracturing fluid leakoff in view of modeling flow in porous media. It is shown how various linear leakoff models have been developed and why a new, radial leakoff concept is necessary for high-permeability fracturing, where the injection time is commensurable to the response time of the reservoir. Using Laplace space methods, the new radial leakoff law is calculated and compared to linear leakoff. For comparison purposes a calibration test executed in high-permeability formation is interpreted using several approaches, namely: linear leakoff+bulk leakoff coefficient; filter-cake resistance+linear flow in the formation and finally, filter-cake resistance+radial flow in the formation. Introduction The polymer content of the fracturing fluid is partly intended to impede the loss of fluid. The phenomenon is envisioned as a continuous buildup of a thin layer (the filter cake) which manifests a resistance to flow through the fracture face. For one of the latest reviews see McGowen and Vitthal. During fracturing, the actual leakoff is determined by a coupled system, of which the filter cake is only one element. The other two important elements are the region invaded by the polymer and/or filtrate and the bulk reservoir itself, containing the original (slightly compressible) reservoir fluid. This work concentrates on the aspect of fluid leakoff which is connected with the bulk reservoir. The methods used are borrowed from the literature on flow in porous media. As usual, we assume that the two wings of a vertical fracture are identical. For modeling purposes we will deal only with one wing. All our variables, including injection rate, i, injected volume, Vi fracture volume, V refer to one wing. (If we want to refer to total injection rate, we write 2i.) By the fracture surface, A, we mean the area of one face of one wing. All these variables may refer to a given time, t, during the treatment. It is important to make a clear distinction between the values of the above variables at any time, t, and at the end of pumping, i.e., at time te. We will use the subscript e if we wish to emphasize that a given value corresponds to the end of pumping. Fig. 1 shows the basic notation on an example of radial fracture. Fluid efficiency, ? is defined as the fraction of the fluid remaining in the fracture: ?=V/Vi As any other state variable it might vary with time. The average width, is defined by the relation The difference of injected and contained volume is the lost volume. The leakoff rate, qL defined here as the volume leaving one wing in unit time, can be calculated from an appropriate leakoff model. Often we assume that the fracture is contained in the permeable layer. Then the whole fracture surface takes part in the leakoff process. If we know the height of the permeable layer, hp we can be more rigorous in taking into account only the actual leakoff surface. Fig. 2 shows how we calculate the ratio of the leakoff surface to the total surface for radial geometry. The ratio, rp is unity for a fracture contained perfectly in the permeable layer and is less than unity if the fracture grows out from the permeable layer. In the case of rectangular fracture shape, rp is the ratio of the "net" height to the "gross" height. The factor is easily incorporated into the derivations, but in the following we do not show rp to increase the readability of the equations. Previous Work Carter Leakoff Model. A fruitful approximation dating back to Carter, Howard and Fast considers the combined effect of the different phenomena as a material property. According to this concept, the leakoff velocity, uL is given by the Carter equation where C L is the leakoff coefficient and t is the time elapsed since the start of the leakoff process. The integrated form of the Carter equation is where V is the fluid volume that passes through the surface AL during the time period from time zero to time t . The integration constant, Sp is called the spurt loss coefficient and is measured in meters. The term Sp can be considered as the width of the fluid body passing through the surface at the very beginning of the leakoff process and the term is the additional width added to it afterwards, up to time t. Formal Material Balance Within the Framework of Carter Leakoff. The Carter leakoff model can be visualized as if a given surface element "remembered" when it had been opened to fluid loss. Every element has its own "zero" time which might vary from location to location on a fracture surface. The application of the model necessitates the tracking of the opening time of the different fracture face elements. The overall volume leaking off from a fracture depends on the time elapsed from the creation of the first element of the surface, on the amount of surface created and also on the distribution of the opening time along the surface.
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- North America > United States > Oklahoma > Anadarko Basin > Carter Field (0.93)
- North America > Canada > Alberta > Howard Field > Bpc Et Al Howard 6-31-79-5 Well (0.93)
Abstract Hydraulic fracturing in medium and high-permeability reservoirs differs significantly from conventional fracturing, since the optimal placement of proppant requires shorter and wider fractures. The optimal fracture dimensions are achieved by executing a tip screenout design technique. Successful application of this technique assumes realistic description of the fluid leakoff process. Fracture calibration treatments (minifracs) have been used to identify leakoff characteristics. In addition to the application of data obtained from such tests (i.e. incorporation into the fracture job design) an increasingly important use is to obtain reservoir engineering parameters, such as formation permeability. This paper is based on two methods, used for the determination of the leakoff parameters from the pressure fall-off stage of a calibration treatment. The first method is the well known technology, which we have called the Nolte-Shlyapobersky method to determine an overall leakoff coefficient. The second method is a modified form of the Mayerhofer et al technique, which attempts to de-couple the two main elements of the leakoff process: the filtercake resistance and the transient flow in the formation. For other methods concentrating more on the pressure fall-off after the fracture closes see References 3 and 5. Field examples will be used to demonstrate the basic steps of the presented methodology. Emphasis is on limitations and possible pitfalls with suggested remedies. A detailed sensitivity analysis is presented which underlines the data quality required to determine reservoir and treatment parameters with any accuracy. P. 493
- Well Drilling > Pressure Management > Well control (1.00)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
Summary Determining the leakoff coefficient from a minifracture pressure decline has become a relatively common industrial procedure. A main assumption of the method, which is often referred to as the Nolte analysis, is a constant leakoff coefficient. Frequently there is no constant leakoff coefficient. The definition of the coefficient is based on a constant pressure differential and a prescribed mode of leakoff. In contrast, the analysis for fracture-pressure decline, introduced by Mayerhofer et al., couples unsteady-state linear flow from the fracture with a varying skin effect at the fracture face, and superposes the leakoff history on the pressure decline. This guarantees a correct rate convolution to account for pressure-dependent fluid loss. The reservoir permeability, fracture-face resistance, and leakoff area can be determined. A comparison of the two methods and their relationship is presented in this paper, using real field cases. We show that the use of modern well-testing log-log diagnostic plots to determine fracture closure pressure is superior to drawing a straight line on a G-function plot or a square-root-of-time plot. Introduction In a series of seminal publications, Nolte introduced a methodology for determining the fracture leakoff coefficient from injection tests. The technique has been widely used, and the determined leakoff coefficient has been incorporated in fracture treatment design procedures. The original technique is based on material balance, and it presumes a mode of leakoff into the formation with the fluid loss dependent on the square root of time. Using this assumption, Castillo introduced a specialized plot where a time-dependent G function forms a straight line with the pressure decline. The straight line is drawn from the independently obtained closure pressure and encompasses all previous pressure points that fall on the straight line. The slope of this straight line is related directly to the leakoff coefficient. The method of analysis and example applications are presented in Refs. 3 and 6. The Nolte analysis and the Castillo specialized plot are exactly equivalent to the Horner construction in pressure-transient analysis. In a similar manner, the Horner analysis also presumes a mode of fluid flow (infinite acting radial flow). A specialized (semilogarithmic) straight line leads to the sought variables (permeability and skin). Yet, investigators of pressure-transient analysis recognized early on that the pressure record of a test rarely, if ever, reflects solely infinite acting behavior at all. To account for the different phenomena, only one of which would lead to a Horner plot, the log-log diagnostic plot of pressure and pressure derivative vs. time is considered the essential first step in modern pressure-transient analysis. In the absence of the characteristic response for radial flow, manifested by a flattening derivative curve, no Horner plot can be constructed. There is no analog in the Nolte analysis for a pattern-recognition exercise, and several attempts have been undertaken to account for deviations such as stress sensitivity (i.e., pressure-transient effects) changing fracture area, dominance (or lack thereof) of the filter cake, and reservoir fluid compressibility. Rules of thumb and approximations have been introduced in attempts to account for the fact that the "assumptions of the basic analysis are seldom met in practice." Nolte et al., in their latest publication, postulated that the filter cake either completely dominates the leakoff or has a negligible effect. For the case of total reservoir control (filtrate invaded plus reservoir) they provided numerical simulations of the pressure history. To determine the slope on the Castillo G plot, they proposed a common reference point at ?pw/?ps+3/4, where ?pw is the net pressure during closing and ?ps is the net pressure immediately after shut-in. This slope was referred to as m3/4. Then the factor kc was used to correct the m3/4 slope. According to Nolte et al., this correction was rarely needed because reservoir control is generally ineffective and, thus, of no practical interest. The Mayerhofer et al. Method Mayerhofer and Economides and Mayerhofer et al. have presented a model that accounts for the filter-cake and reservoir response, allowing for the superposition of the injection history, filter-cake deposition, and associated rate convolution. Their solution isEquation 1 The normalized resistance, RD, is simply given byEquation 2 Although Eq. 1 may appear complex, it is in fact relatively easy to track as outlined in detail in Ref. 10. Central to the method is the use of log-log diagnosis, a standard tool in pressure-transient analysis. A successful addendum is the use of the rate-normalized plot (RNP), also a standard in pressure-transient analysis, which allows the detection of trends by deducing rate-transient effects. Mayerhofer et al. have shown in a series of simulations that reservoir dominance would be manifested by a half-slope straight line on the log-log RNP and RNP-derivative plots, whereas filter-cake dominance would appear on the RNP derivative as an increasing trend but of decreasing slope approaching the half-slope straight line. The larger the fracture-face resistance, the more pronounced its dominance on the pressure decline and the steeper the slope of the derivative plot would be. Interestingly, Nolte's presumption of the leakoff rate depending on the square root of time but with the filter cake dominating does not form a half-slope straight line. Only reservoir dominance, i.e., linear flow from an infinite conductivity fracture into the reservoir, would result in a half-slope straight line on the log-log diagnostic plot.
Abstract Starting from the original concept proposed by Carter, Howard and Fast, this paper reviews the description of fracturing fluid leakoff in view of modeling flow in porous media. It is shown how various linear leakoff models have been developed and why a new, radial leakoff concept is necessary for high-permeability fracturing, where the injection time is commensurable to the response time of the reservoir. Using Laplace space methods the radial leakoff law is calculated and compared to linear leakoff. For comparison purposes a calibration test executed in high-permeability formation is interpreted using several approaches, namely: linear leakoff + bulk leakoff coefficient; filtercake resistance + linear flow in the formation and finally, filtercake resistance + radial flow in the formation. Introduction The polymer content of the fracturing fluid is partly intended to impede the loss of fluid. The phenomenon is envisioned as a continuous build-up of a thin layer (the filtercake) which manifests a resistance to flow through the fracture face. For one of the latest reviews see McGowen and Vitthal. During fracturing, the actual leakoff is determined by a coupled system, of which the filtercake is only one element. The other two important elements are the region invaded by the polymer and/or filtrate and the bulk reservoir itself, containing the original (slightly compressible) reservoir fluid. This work concentrates on the aspect of fluid leakoff which is connected with the bulk reservoir. The methods used are borrowed from the literature on flow in porous media. As usual, we assume that the two wings of a vertical fracture are identical. For modeling purposes we will deal only with one wing. All our variables, including injection rate, i, injected volume, Vi, fracture volume, V refer to one wing. (If we want to refer to total injection rate, we write 2i.) By the fracture surface, A we mean the area of one face of one wing. All these variables may refer to a given time, t during the treatment. It is important to make a clear distinction between the values of the above variables at any time, t, and at the end of pumping, i.e, at time te. We will use the subscript e if we wish to emphasize that a given value corresponds to the end of pumping. Figure 1 shows the basic notation on an example of radial fracture. Fluid efficiency, is defined as the fraction of the fluid remaining in the fracture: = V/Vi. As any other state variable it might vary with time. The average width, w, is defined by the relation V = Aw. The difference of injected and contained volume is the lost volume. The leakoff rate, qL defined here as the volume leaving one wing in unit time and can be calculated from an appropriate leakoff model. Often we assume that the fracture is contained in the permeable layer. Then the whole fracture surface takes part in the leakoff process. If we know the height of the permeable layer, hp, we can be more rigorous in taking into account only the actual leakoff surface. Figure 2 shows how we calculate the ratio of the leakoff surface to the total surface for radial geometry. The ratio, rp, is unity for a fracture contained perfectly in the permeable layer and is less than unity if the fracture grows out from the permeable layer. In the case of rectangular fracture shape rp is the ratio of the "net" height to the "gross" height. The factor is easily incorporated into the derivations, but in the following we do not show rp to increase the readability of the equations. Previous work Carter Leakoff Model A fruitful approximation dating back to Carter, Howard and Fast considers the combined effect of the different phenomena as a material property. According to this concept, the leakoff velocity, uL, is given by the Carter equation: (1) P. 135^
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- Well Drilling > Pressure Management > Well control (1.00)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
Summary Fracture-calibration pressure decline has been used for determination of the leakoff coefficient, a bulk variable describing the process of fluid influx into the reservoir, normal to the created fracture face. In this work, the fluid loss is modeled in terms of the controlling mechanisms: flow through the filter cake, the invaded zone, and the reservoir. A rigorous model describes unsteady-state fluid flow from fractures of varying area into the formation, with the filter cake considered as a time- and rate-dependent skin effect. The injection history is superposed on the pressure decline. This work provides a straight-line technique for determination of reservoir permeability and fracture-face resistance. Log-log diagnostic plots provide the means to recognize visually whether the transient response is dominated by flow in the reservoir or at the fracture face. We found that the pressure transient very frequently is dominated by the flow in the reservoir rather than through the filter cake. The reservoir permeability (an essential value for fracture design that is usually not available) can be estimated, while the model captures all trends of the falloff-pressure variation. Introduction The fracture-calibration treatment, also called an injection test or "minifracture," frequently is conducted before the main stimulation treatment. For the injection test, the fracturing fluid intended for the main treatment is pumped at a constant rate of a sufficient magnitude to achieve fracturing pressure. After several minutes (usually 20 to 30) the pumps are shut off and the bottomhole pressure declines as the fracture closes. During pumping, while the fracture volume grows, some of the fracturing fluid leaks off into the formation. After pumping ends, the fluid leakoff into the formation continues until the fracture is closed. Material balance, coupled with a model of propagation, permits estimation of the rate of fluid loss during pumping. The behavior of the pressure decline after the end of pumping has been used to estimate the leakoff (fluid-loss) coefficient, with techniques pioneered by Nolte. Castillo, using Nolte's G-function for modeling the pressure-decline behavior, developed the straight-line plot of the G-function vs. pressure. The slope of this curve is used for the computation of the leakoff coefficient that is independent of pressure.
Abstract A potentially important component of the total pressure drop from a fracture into a formation during hydraulic fracture stimulation is that across the polymer invaded zone. Following experimental work with long cores (up to 60 cm-long) and with permeabilities up to 5 md it was determined that the polymer invasion for a zirconate- crosslinked HPG-system has not particular significance and is thus negligible compared to the very dominant filtercake. The observed early- and late-time filtration phenomena were described using a numerical cake layer model and verified by experimental measurements. It is shown that the early-time "hyperbola" is not a result of polymer invasion but is caused by a changing pressure differential across the filtercake at the beginning of the filtration experiment. Introduction Fluid leakoff during hydraulic fracturing is controlled by a pressure difference, which is the sum of the pressure drops across the filtercake, across the polymer invaded zone, through the filtrate invaded zone (if displacement effects are significant) and in the reservoir. The latter is controlled by the transients of linear filtrate flow. In the field the pressure drop components in the vicinity of the fracture face can be treated as a skin effect. These near face components are the filtercake and the polymer invaded zone. (Displacement effects are not considered here.) Their rate, time and stress-dependent properties are especially important since the skin effect component is likely to change accordingly during the fracturing operation and may have an effect on the pressure response. Their contribution can be thoroughly investigated in the laboratory, and thus, they may be included in the transient solution for varying injection (leakoff) rates through an infinite-conductivity fracture during a fracture calibration treatment. The component filtercake was analyzed in a series of fluid-loss experiments by Mayerhofer, Economides and Nolte for zirconate-crosslinked HPG fracturing fluids and by Zeilinger, Mayerhofer and Economides for borate-crosslinked fluids. Especially, the stress-sensitivity of crosslinked polymer filtercakes was investigated in detail by imitating the pumping and closure phase of a fracture treatment by continuously increasing and, subsequently, decreasing the pressure. These varying pressure differentials must account for the superposition of additional cake deposition and influence the stress-sensitive properties of the already deposited cake. The concept of hydraulic filtercake resistance was introduced. It was found that viscoelastic theory can provide an adequate background to explain the observations of pressure-dependent leakoff. The second component, the influence of polymer invasion at the fracture face for crosslinked fracturing fluids was not considered adequately in the literature, although there is some indication of possible effects. Additionally, it was shown in Ref. 3 that for non-crosslinked HPG-fluids there exists a limit core permeability between filtercake-controlled filtration and a non-Newtonian filtration. The decoupling of polymer invasion and its contribution to the overall flow resistance for long cores between 1 to 5 md permeability for a zirconate-crosslinked HPG fluid was the scope of this work. It was done using permeability measurements and pressure transducers positioned near the core face. P. 741^
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (1.00)
- Well Completion > Hydraulic Fracturing > Fracturing materials (fluids, proppant) (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Well Drilling > Pressure Management > Well control (0.95)
SPE Members Abstract Static leakoff experiments with filter paper, using borate- and zirconate-crosslinked hydroxypropylguar (HPG) fluids, have resulted in practically the same leakoff coefficients. This is in contrast to previous, dynamic, leakoff tests suggesting that borate-crosslinked fluids perform better. Within the temperature range where the polymers are thermally stable (200 degrees F), there were no temperature effects after correcting for the filtrate viscosity. Under constant pressure differentials, both fluids exhibited compressible filtercake behavior with the leakoff coefficient approximately proportional to powers of 0.2 for borates and 0.17 for zirconates. When pressure was increased, imitating pumping and, then decreased, imitating closure, characterization of the pressure-dependent fluid loss using the leakoff coefficient as the descriptive parameter indicated a fluid-loss hysteresis for both fluids. parameter indicated a fluid-loss hysteresis for both fluids. During the pressure increase from 0 to 1400 psi both fluids showed a two-range response: incompressible behavior at low pressures and compressive behavior at higher pressures. In this range, the leakoff coefficient was practically constant. A comparison of the stress-sensitive properties has shown that while zirconate filtercakes exhibit decidedly viscoelastic properties, borate filtercakes are merely elastic. This can be properties, borate filtercakes are merely elastic. This can be attributed to the differences in the nature of the bonding of these polymers. Finally, filterpaper and core experiments, done with noncrosslinked fluids, have shown no filtercake-type behavior for a large range of core permeabilities, but rather a viscous flow dependent on porous medium characteristics. Introduction Fracturing fluids are expected to perform two main tasks: to propagate a fracture and to transport the proppant. This is propagate a fracture and to transport the proppant. This is accomplished by the build-up of viscosity at reservoir temperature and by controlling the leakoff through the creation of a filtercake on the fracture walls. The viscosity function can be accomplished with noncrosslinked fluids for low-temperature applications or with crosslinked polymeric solutions for higher temperature reservoirs. Borate- and zirconate-crosslinked fluids have found wide applications in the petroleum industry. The leakoff control function requires further investigation. Nolte, in significant publications has presented techniques to evaluate the leakoff mechanism by the introduction of calculation procedures for the leakoff coefficient. In addition, he determined the impact of inadequate leakoff control. Mayerhofer, Economides and Nolte performed both experimental and theoretical studies for a more fundamental understanding of the interaction of filtercake behavior and fracturing fluid loss for zirconate-crosslinked HPG fluids (ZXL). The effects of temperature, polymer loading, pressure and pressure path were investigated. That work has pointed out pressure path were investigated. That work has pointed out the necessity for reconsidering the pressure dependence of compressible filtercake fluid loss, especially for varying pressures that simulate the pumping and closing phase of a pressures that simulate the pumping and closing phase of a fracture treatment. Temperature effects were found to be related only to the viscosity of the filtrate. P. 201
Abstract Interpretation and modeling of filtercake leakoff experiments are presented. Crosslinked polymer fracturing fluids were used under static conditions. Relationships between ratios of leakoff coefficients and polymer loadings were determined. These can lead to the calculation of any value if one is obtained experimentally. It was determined that the filtercake behavior deviated from the square-root of pressure dependence for an incompressible cake. Experiments, using a constant pressure differential across the cake, indicate relationships pressure differential across the cake, indicate relationships between leakoff coefficient and pressure raised to powers approximately 0.25 or less depending on the pressure value whereas previous work reported a single proportionality for all pressures. Since pressure varies continuously throughout the pumping and closing of a fracture treatment it was imitated experimentally resulting in different pressure differentials at different times. The observed behavior is explained introducing the concept or filtercake resistance. Data interpretation for changing pressures indicates an apparent fluid-loss hysteresis, characterized by the behavior of polymer cakes subjected to changing pressures and by additional polymer cakes subjected to changing pressures and by additional cake build-up. A more fundamental interpretation is provided from the theory of viscoelasticity. The hydraulic provided from the theory of viscoelasticity. The hydraulic filtercake resistance, proportional to a rate-normalized pressure difference, leads to a stress-sensitive skin-effect, exactly analogous to similar situations in pressure transient analysis. Introduction Substantial experimental and theoretical work has been conducted and reported for fracturing fluid leakoff. Since filtercake properties play an important role in fracturing fluid loss, different leakoff control mechanisms were described and modelled for downhole conditions. In a series of experiments, Mayerhofer, Economides and Nolte investigated the effects of temperature, polymer loading and pressure dependence of static filtercake fluid loss. Special emphasis was laid on describing effects of pressure and pressure path. The pressure-sensitive behavior, especially for varying pressures, simulating the pumping and closure phase of a fracturing treatment, have not been subjected to a rigorous analysis in the past. In contrast to the common procedure of filtration under constant pressure differentials, procedure of filtration under constant pressure differentials, varying pressure differentials must account for the superposition of additional cake increase and stress-sensitive effects of the prior deposited cake. The definition and the common method of calculating the leakoff coefficient is not appropriate for interpreting and explaining effects caused by changing pressures. The concept of hydraulic filtercake resistance was pressures. The concept of hydraulic filtercake resistance was introduced and found to be more suitable for the interpretation of the process. Empirical proportionalities beween leakoff coefficients and pressure are sufficient for generalized interpretation and field pressure are sufficient for generalized interpretation and field application but lack any fundamental explanation of stress induced phenomena. Polymer filtercakes as solid polymers and polymer solutions, have viscoelastic properties. This work provides a new approach to modelling and explaining pressure-dependent fluid loss using viscoelastic theory. pressure-dependent fluid loss using viscoelastic theory. All experiments were conducted with a high-pressure, high temperature filter press commonly used for static fluid-loss testing. A zirconate crosslinked HPG-polymer solution, without any fluid-loss additives and prepared according to standard industry procedures, was used as the fracturing fluid system. Filterpaper was selected as porous medium in order to isolate the wall-cake effects and guarantee a high experimental reproducability. Filtrates were continuously collected and recorded. P. 557
Abstract Kick control for ultradeep boreholes has certain important distinguishing features from the kick control for more "normal" depth wells. The annular pressure profile vs. time is different in the two cases because the initial shut-in pressure in the ultradeep holes is large and sometimes it is the largest during constant bottomhole pressure (CBHP) kick control, resulting in small gas expansion effects. In shallow holes, the lower initial shut-in pressure and low pressure level (compared to ultradeep borehole annular pressures) throughout the kick control operation results in significant gas expansion. Therefore the annular pressure during the kick control may be substantially higher than the shut-in pressure. Thus, the critical period between the two cases happens at a decidedly different time. Kick simulation for ultradeep holes is presented in this paper for different CBHP methods using waterbase muds in any borehole geometry. The worst case, that of a sudden influx of a gas slug is used. The results are profiles of surface annular pressures, identifying the largest pressure value at each depth during the kick circulation with the CBHP methods. This allows the quantification of the load on a borehole during controlled CBHP kick-killing-operations at any depth. Introduction Recent work (Lorbach and Schoffman) involved research for kick control in ultradeep wells in metamorphic formations done for the German Continental Deep Drilling Project (KTB-project). This project was initiated for testing earth stresses, geophysical rock parameters and fluid movements at great depth. The main borehole is drilled in nearly impermeable metamorphic rocks where usually no kick situation, caused by gas or fluid influxes, may be expected. During the drilling of the 4000 m (13,123 ft) deep pilot hole (finished in Sept. 1990) four drill stem tests were done, and one of them detected a small inflow (Rischmuller). Nearly at the planned depth of the pilot hole 3997 m (13,114 ft) a major influx occured after the pilot hole 3997 m (13,114 ft) a major influx occured after the drilling ceased. The initial differential pressure of about 40 bar (580 psi) induced a 9 m3 (56.6 bbl) inflow of saline water with gas content. That unexpected occurence pointed to the need of considering kick situations even in "metamorphic" formations. The probable pressure loads in the well were determined to allow the use of existing kick control methods, in the worst case of an influx of gas at the planned target depth of 10,000 m (32,808 ft). [The optional depth is 12,000 m (39,370 ft). This work presents calculations up to 14,000 m (45,932 ft). The probable presents calculations up to 14,000 m (45,932 ft). The probable pressure loads in the ultradeep well were related to known pressure loads in the ultradeep well were related to known geological and mechanical pressure limits at the borehole. Some of the considered safety limits for an ultradeep well were: Maximum allowable shut-in time for a given pore pressure as a function of the openhole length and mud density and the maximum pressure at each depth Maximum allowable pore pressures for a controlled kick operation with one of the CBHP methods depending on the gas influx volumes Allowable kick sizes as functions of different equivalent pore pressures, mud densities and openhole depths Additional considerations were the forming of hydrates in natural gases (0.6 gravity) during the expansion of the gas at the choke. The problem can be easily overcome by heating the choke line to maintain the inlet temperature. P. 265
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