Long-term zonal isolation provided by cement is a crucial task in the life of oil and gas wells. However, significant number of primary cementing jobs experience problems, particularly in highly deviated wells, extended reach wells, and wells prone to severe washouts. Primary cementing efficiency has attracted more attention since the development of shale gas industry and the Macondo blowout in the Gulf of Mexico. Sustained casing pressure reported in some of the wells in Marcellus shale play and the root of several blowouts is attributed to the quality of cement job. Therefore, studies on the performance of cementing operations are essential in restoring the public opinion on petroleum industry by addressing problems that have major social, environmental and economical consequences besides the technical interest.
Casing is prone to deviate toward the bottom of the well especially in horizontal wells. This eccentric annular space leads to annular velocity disturbance in favor of wider region of the annulus. Finally, part of the narrow section of annulus would be left un-cemented. The bypassed mud is a potential path for the formation fluid communication with other formations or to the surface. Poor cementing can affect the hydraulic fracturing job as well.
This paper reports on a comprehensive three-dimensional time-dependent computational fluid dynamics (CFD) model developed to account for dominant parameters affecting the mud displacement process in horizontal wells. Parameters such as casing eccentricity, cement yield strength, cement plastic viscosity, the density difference between mud and cement, pumping rate and washout are studied. The effects of the first three parameters are addressed in this paper.
Current best cementing practices have deficiencies in providing excellent cementing efficiency. Therefore, a novel technique, using Magneto-Rheological (MR) fluid, is also proposed to improve the displacement efficiency. MR fluid can act as a plug in the wider region of annulus under a magnetic field applied through the casing. Consequently, flow will be directed to the narrower annular region that could not be cemented or cleaned otherwise. The results are appealing and further study on the application of MR fluid in petroleum industry is suggested.
Proppants are essential to the success of most hydraulic fractures and often account for the overwhelming cost of the treatment. Both the mass of proppant and the selection of the right type of proppant are important elements in gaining the highest Net Present Value (NPV).
It has been generally believed that in the lower closure stress environment (below 6,000 psi, i.e., shallow reservoirs), natural sands such as Brady and Ottawa are appropriate as proppants and, for the same mesh size, they provide essentially the same permeability. A commonly accepted notion is that manmade proppants (such as ceramics) should be applied at higher closure stress environments, invariably, deeper reservoirs.
The characteristics of most shale plays are very different, mainly as regards to the rock stiffness, exemplified by the Young's Modulus, stress anisotropy/isotropy and the existence of natural fracture network. Fracture strategies in shale formations are very different. This study presents fracture designs based on three types of proppants for shale formations: Brady sand, Ottawa sand and ceramic. Permeability tests and crush tests under certain pressure range are done to determine experimentally the dimensioned fracture conductivity. A fracture optimization p-3D model is used to maximize well performance by optimizing fracture geometry, including fracture half length, width and height. Reduced proppant pack permeability is compensated by larger width. Non-Darcy effects in the fracture are also considered for gas reservoirs. Post-treatment well performance is then estimated, using the optimized well geometry, leading to cumulative production over the well life. NPV analysis is employed as the criterion to select the best proppant for the job. Finally, the completion and production data from example wells will be analyzed for comparison purpose.
In this work, we try to correct the prejudice that natural sand proppants cannot be applied to deeper reservoirs by showing NPV study results that are superior to those of manmade proppants. Keeping stimulation costs down, natural sands proppants have a much larger range of applicability than previously thought.
To date the most prolifically producing geologic structure in Montana's Bakken formation is the Elm Coulee Field. While the first wells drilled into this structure 35 years ago had some success, significant production did not occur until horizontal wells were drilled starting in 2000. The Bakken Petroleum System consists of three members, the Upper Bakken, shale, the Middle Bakken, silty dolostone, and the lower Bakken, siltstone. Production of the Bakken Petroleum System is extremely difficult to predict with standard reservoir models. Thus, the decision to understand initial production through geologic drivers was made. An investigation of the implications of ancient geology on what is believed to be specific areas of micro-flexures in and around the major geologic structures in Montana's Bakken three-member formation will form the basis for a Bakken production model.
The challenge in modeling the Bakken's production, both initial and subsequent, stems from the unusual history matching problem of either extremely high initial production with an unpredictably low subsequent production or surprisingly low production to begin with. The authors believe this phenomenon is due to broaching a series of micro-flexures and draining them, resulting in high initial production and unsustained subsequent production, or missing these structural flexures all together resulting in low production. This paper will investigate the genesis of Montana's Bakken micro and macro fractures with the ultimate goal of simulating and thus predicting production for wells currently producing in the area.
Shale gas reservoirs are quickly becoming an important source for natural gas. Historically, the shale basins were not looked at for economic production because of their very low permeability values. However, with the use of hydraulic fracturing combined with horizontal well completions, shale plays have become economically producible, especially the Marcellus shale. In order for these wells to produce, hydraulic fracturing must be employed. This technique makes production viable, but it also changes the in-situ stress field, through the fracture itself and the increased production that occurs. This change in the stress field can damage the wellbore and compromise the integrity of the casing.
In this study, geomechanic and reservoir models were constructed using commercially available software to study the change of the stress field in the formation. The parameters of the hydraulic fracture were varied to study the effects of the hydraulic fracture half length and the fracture permeability on the stress field, specifically near the wellbore.
This study develops a process that determines the critical hydraulic fracture parameters and quantifies their impact on the EUR by combining reservoir simulation with probabilistic analysis methods. The process is verified by a real field case example in a tight gas reservoir. The final product can be applied to other unconventional reservoirs to ultimately maximize revenues by planning superior fracturing operations and optimizing well spacing.
A detailed dual porosity, 1 section reservoir model was created and history matched to model the flow mechanism. A fine layered (2-3ft) geostatistical model was utilized in simulation without upscaling. The dual porosity formulation enabled the simulation model to represent the hydraulic fracture - matrix interaction properly so that the flowback and formation water production could be matched also.
During the history matching phase, the parameters that control the impact of hydraulic fractures on the recovery were identified as follows:
In this work, an internal proprietary technology that creates a response surface for the combination of the parameters defined above was utilized. This technology utilizes Experimental Design, Response Surfaces and Constrained Monte Carlo. The history matched simulation model was automatically modified to create the necessary cases to calculate a multi-dimensional response surface. The created response surface was then used to do Monte Carlo simulations to create P10 to P90 probabilities of the total gas production (EUR).
The results of the study allowed us to understand not only the mechanisms operating in the reservoir being studied, but also the required hydraulic fracture parameters (ranges) to achieve a given EUR of a specific probability. The same algorithms were then be used to predict the future performance of other well spacing patterns and hydraulic fracture job sizes.
A new algorithm for processing cross-dipole acoustic log waveforms in formations with horizontal transverse anisotropy (HTI) has been developed. Conventional algorithms for anisotropy processing of cross-dipole acoustic waveforms minimize an objective function whose parameters are the azimuth angle of a reference (usually X-dipole) transmitter relative to the fast principal flexural-wave axis of the HTI formation and the fast and slow shear wave slowness. Minimizing the objective function with respect to all the parameters provides the desired anisotropy angle and the amount of anisotropy. It is common to minimize the objective function over all the parameters using a numerical search method (such as very fast simulated annealing) or by evaluating the objective function on a fine grid in the parameter space (i.e., brute force); however, this is never necessary with cross-dipole waveforms. It is possible to derive equations for the angle of the X-dipole relative to the fast principal flexural-wave axis that can be solved analytically as a function of the other parameters.
Analytic computation relative to numerical computation provides several advantages. First, the angle found at any given point in the auxiliary parameter space is a mathematically exact global minimum (up to computer precision) with respect to the angle of the objective function at that point in the auxiliary parameter space. This is not guaranteed with a numerical-search algorithm. In addition, studying the objective function at the minimizing angle with respect to the remaining auxiliary parameters can provide insight as to the best means of minimizing the objective function with respect to the remaining parameters. This paper uses synthetic and field data to demonstrate the advantages of this new algorithm.
Sand jet perforating (SJP) is currently gaining acceptance in unconventional resource plays where horizontal development has shown the need for methods to perforate these formations. This paper examines a new design for a tool assembly that allows fluid to flow through the sand jet perforator while operating a PDM (Positive Displacement Motor) with a mill or cutting tool. In multi-stage fracturing operations where through tubing mills are often used for cleanout runs prior to perforating, the flow isolation tube assembly can be added to the tool string to achieve cleanout in conjunction with perforating in one trip. The design allows an operator to isolate the SJP tool, which is resident in the work string and perforate casing and formation, then restore flow to the PDM to continue operations such as milling or further well cleanout- all in a single trip.
Discussion of the assembly design process and laboratory testing results are included as a part of the paper and will analyze the effectiveness of the technology and tools as well as the economic impact of this type of program. Economics includes process costs as well as cost savings from support equipment that is gained by the combination of operations. The program's translation to other shale plays like the Marcellus, Bakken, and Haynesville are also discussed.
Shale plays dominate domestic drilling as well as production today and being able to combine completion operations to save both time and money is of interest to all producers. Using the flow isolation tube assembly will save cost and wear for coil tubing units as well as costs for other surface support equipment. Finally, safety is increased and the quality of the perforations is improved with SJP; giving a better perforating job for a lower cost.
Bearings used in directional drilling operate in conditions which are significantly more severe than those found in traditional industrial applications. One of the most difficult conditions mud motor thrust bearings encounter is misalignment, which is amplified in bent housing motors. The purpose of fixed bend and adjustable bend motors is to build a desired angle in a well. To accomplish this build rate the housing is manufactured with a purposely bent joint which typically deviates between 1 to 3 degrees from the rest of the bottom hole assembly. Radial bearing wear creates uneven thrust bearing loading due to angular misalignment. Uneven loading in ball bearings results in significantly higher Hertzian stresses which reduces bearing life and reliability.
Uneven loading in diamond, or PDC, bearings results in premature failures due to increased stresses and chipping. The proposed paper will demonstrate that a thrust bearing design incorporating independent and resiliently mounted thrust pads operating against a continuous rotating bearing surface is effective at handling misalignment in bent housing mud motors.
Fracture conductivity in many hydraulic-fracturing treatments can be inadequate. It is greatly affected by the concentration of the packed proppant in the fracture. Higher concentrations yield higher conductivity by virtue of a wider fracture. However, there are practical limitations to the amount of proppant that can be placed into any particular reservoir, and therefore production is often conductivity limited.
An alternate approach to achieve high conductivity is to create a fracture by placing well-distributed, low-density particles characterized by a proppant concentration less than 0.1 lbm/ft2. Low particle concentrations result in fractures that have high porosity and are fundamentally different from fractures with packed beds of conventional proppants.
In this paper, the theoretical basis for the conductivity of these fractures is presented. A 3-D model has been developed to simulate high-porosity fractures created with these particles. Test data used to refine the model can be used to predict the conductivity of the fracture based on the porosity level, the closure stress, and the material properties.
Production data from two application areas in North America are shown to highlight the benefits of using this type of fracturing proppant.
A screening life cycle analysis (LCA) is included to evaluate and highlight the beneficial attributes of using a low-density proppant to achieve fractures with high conductivity. The LCA considers the impact of logistics and fracture design on the environment.
Casing drilling is used as an alternative to conventional drilling with drillpipe in order to reduce non-productive time. The smaller annular space in casing drilling elevates the annular pressure loss considerably at similar flow rates in conventional drilling. Consequently, the Equivalent Circulating Density (ECD) is more affected by annular drilling fluid dynamics in casing drilling than the conventional drilling. The higher ECD experienced in casing drilling brings concerns about exceeding fracture gradient which can lead to induced lost circulation. However, several field observations demonstrate successful application of casing drilling in combating lost circulation and strengthening the wellbore.
Smearing effect theory backed by smaller cuttings at the shale shaker, eccentric casing wear, and discrepancy between analytical and field measurements are three main evidences for potential significant eccentricity in casing drilling operations. This paper demonstrates the inherent eccentricity of casing drilling as one of the parameters that controls the annular pressure losses. Eccentricity reduces the velocity in the narrow section of annulus. Similarly, it reduces the annular pressure losses considerably. In addition, controlling the fluid rheological properties as well as the flow rate are recommended to manage the casing drilling hydraulics. This comprehensive study of pressure loss and velocity profile at various annular sizes can help analyzing several field observations and designing the hydraulics of drilling operations.