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Siebrits, E. (Schlumberger Dowell) | Elbel, J.L. (Schlumberger Dowell) | Detournay, E. (Univ. of Minnesota) | Detournay-Piette, C. (Itasca Consulting Group, Inc.) | Christianson, M. (Itasca Consulting Group, Inc.) | Robinson, B.M. (S.A. Holditch & Assocs.) | Diyashev, I.R. (S.A. Holditch & Assocs.)
Abstract Refracturing in a plane normal to that of the initial fracture can be beneficial in tight gas fields, even when the initial fracture is deeply penetrating and highly conductive. The magnitude of the benefits will depend on the timing and the penetration of the secondary fracture. The Gas Research Institute has sponsored a project that addresses the conditions required for maximum benefit and the validation of the process with a field trial. This paper addresses the theory and conditions required for maximum benefit. P. 17
Abstract The success of conventional fracturing (using non-reactive fluids to carry proppant) and acid fracturing is dependant on both the creation of effective fracture conductivity and fracture penetration (fracture half-length). With acid fracturing, nonuniform acid-etching (or differential etching) of the fracture face creates lasting conductivity as long as stable points of support (asperities) exist along the etched fracture length. These hold the channels open and connected to the wellbore following fracture mechanical closure. However, both field experience and laboratory work have shown that even fairly competent carbonates soften and creep under closure stresses after contact with acid, thus, potentially resulting in poor retention of acid-etched fracture conductivity. Preservation of fracture conductivity becomes even more challenging in case of high effective closure pressure. Furthermore, acid fracture conductivity is dependant on surface etching patterns, which are determined by uneven permeability and mineralogy distributions. Therefore, a very clean, homogeneous isotropic carbonate may not be a good candidate for acid fracturing since a fairly uniformly etched fracture might close completely at bottomhole producing pressures. Also, carbonate formations with more than approximately 30 percent insoluble components are generally not good candidates because overall acid-etched fracture conductivity may be impaired due to low solubility and also the release of insoluble materials may tend to plug any conductive etched patterns created by the acid. The effective length of the acid-etched fracture is limited by the distance the acid can travel along the fracture and adequately etch the fracture faces before becoming spent. When acid fracturing, the etched length, not the hydraulic length, is considered the effective fracture length. Effective acid penetration will most often be shorter than any proppant placement (due to often high and increasing leak-off rates with time, and high reaction rates, especially at elevated temperatures). An indeed rare, but in theory, powerful well stimulation technique is the combination of acid fracturing (i.e., creation of a hydraulic fracture using reactive acid fluid) with proppant (CAPF) to provide permanent conductivity. Unless proppant is squeezed into the acid fracture before the end of the job, the conductivity of an acid fracture is vulnerably retained pending the stability of asperities all along the height/length of the fracture. Thus, the desire to include proppant in fracture acidizing treatments is conspicuous (but not limited to) "clean" carbonates (exhibiting uniform mineralogy and permeability), carbonates at high effective stress conditions, "soft" carbonates of any permeability (excluding high porosity chalks), low temperature dolomites (with low reaction rates) and together with organic acids where small and vulnerable etched-fracture widths are prevalent. Also, intuitively, effective fracture half-length may be extended if acid (or non-reactive fluids) can transport proppant beyond the etched penetration length "all the way" to the hydraulic tip of the fracture or even extend the hydraulic length for typical short acid fractures. A methodology proposed by Dowell more than three decades ago "Maximum Conductivity Stimulation" (MCS) is probably the first discussion of the idea of combining acid with proppant fracturing. However, the idea did not establish roots in the oil and gas industry for reasons discussed in this paper. Clearly, one missing ingredient was the lack of today's state of- the-art modeling tools for determining suitable applications and procedures. This paper presents and uses a recently developed planar 3D, gridded, FEM (finite element method) multi-layer (with varying percent of limestone/dolomite including non-reactive layers) acid fracturing model. This model fully couples rock mechanics (fracture width and propagation), matrix and natural fracture fluid loss (and effects of acid and non-acid gel fluid stages to increase and reduce fluid loss, respectively), acid reaction/acid diffusion, fluid flow, and proppant/acid transport into a single solution. Such a capability is unique at this time, and, in general, only a 3D gridded model is capable of such simulations due to the complex interactions. Case histories are examined in this paper as possible targets for CAPF. The extraordinary simulation results from modeling of this combined process and its impact on well productivity are discussed.
Designing an acid-fracturing treatment is similar to designing a fracturing treatment with a propping agent. Williams, et al. presents a thorough explanation of the fundamentals concerning acid fracturing. The main difference between acid fracturing and proppant fracturing is the way fracture conductivity is created. In proppant fracturing, a propping agent is used to prop open the fracture after the treatment is completed. In acid fracturing, acid is used to "etch" channels in the rock that comprise the walls of the fracture.
Cramer, D.D., The Western Co. of North America
Madison carbonates in the Williston Basin vary widely in productive capacity. Often, wells achieving low oil production rates after acid stimulation respond favorably to a proppant fracturing treatment. This has occurred most frequently in fractured reservoirs with limited matrix permeability (less than 0.1 md).
The study are consists of nine contiguous fields 10-15 miles south of the town of Williston North Dakota. Madison reservoir development was initiated in 1980 and has proceeded steadily. A variety of stimulation techniques have been implemented with proppant fracturing emerging as a frequently used method. This paper reviews and evaluates area reservoir characteristics and completion and stimulation practices. Also included is an analysis of the reservoir behavior during proppant fracturing treatments and of the ensuing proppant fracturing treatments and of the ensuing production response. In most cases, proppant production response. In most cases, proppant fracturing techniques have provided better productivity increases than large-scale acidizing productivity increases than large-scale acidizing and lower than normal decline rates.
The "Elk" area is an informal name for an oil play in the northwest McKenzie County Dakota play in the northwest McKenzie County Dakota (Figure 1). Development is concentrated in a four township section (T151 and 152N - R101 and 102W); however, these borders have been extended (Figure 2). Production from Red River, Stonewall Interlake, Duperow, Nisku, and Madison intervals have been established from the nine contiguous fields (Table 1, Figure 2) that comprise the area. There is a broad level of involvement as 21 companies participate as lease operators.
Madison reservoir development in each field evolved as a consequence of deeper drilling objectives. Seismic data was gathered in search of Paleozoic "highs". Prospects were identified and Paleozoic "highs". Prospects were identified and several wells were completed as Red River producers. It was apparent from mud logs, drill producers. It was apparent from mud logs, drill stem tests (DST), open hole logs and subsurface measurements that structural influence carried through to the Mississippian intervals. The recompletion of the Superior Oil Donald Link #1 in the Mission Canyon formation signalled the beginning of a trend that is in full swing today. Seventy percent of all wells in the study area are now completed in the Madison.
Problems encountered by area Madison operators include steep production decline rates and low initial oil yields. Improvement of this situation has been achieved from stimulation treatments. This paper provides information on overall reservoir and production characteristics and how they relate to the different types of stimulation treatments performed. Data is then presented showing the superiority of proppant presented showing the superiority of proppant fracturing techniques in achieving "long term" productivity increases. productivity increases. RESERVOIR DESCRIPTION
Madison sediments consist of evaporitic and carbonate rock extending from the base of the Kibbey Sand to the top of the Bakken Shale. Within this 1,600 to 1,700 foot sequence, production is established in the Ratcliffe production is established in the Ratcliffe sub-interval and in Upper, Middle, and Lower Mission Canyon sub-intervals (informally named). These sub-intervals are located between the base of the last Charles salt and the top of the Lodgepole formation, where thickness range from 722 to 779 feet in the study area. The top 40 to 50 feet consist of alternating layers of anhydrite and non-porous dolomite. These have formed an effective seal for Ratcliffe reservoirs below.
The Ratcliffe sub-interval consists of 85 to 100 feet of alternating limestone, dolomite, and anhydrite beds. The bulk of porosity development is found in the lower 20 to 40 feet of this zone, although a four foot porous dolomite is usually encountered at the top. In the better Ratcliffe wells, porosity values range from 7 to 13% and compensated neutron density (CND) log response indicates that the rock is dolomitic.
Abstract Oil and gas well completion and stimulation can be accomplished by the use of bullet impact or the impact resulting from a shaped-charge jet. Jet penetration can have the distinct advantage of leaving a clean and unobstructed hole. The phenomena associated with charge liner collapse are generally understood. The target response to the action of the jet and the resulting fractures are areas which required more research. A high speed Cordin Framing Camera was used to photograph the jet penetration and fracture formation in models made from Plexiglas. The jet penetration rate, fracture orientation, and Plexiglas. The jet penetration rate, fracture orientation, and radial fracture propagation velocity could be determined with the use of this technique. In addition to Plexiglas, the fracture phenomena in rock under standard temperature and pressure phenomena in rock under standard temperature and pressure conditions were investigated by the authors. During the jet penetration process, the compressive wave pulse will decrease in magnitude and the manner of failure in the pulse will decrease in magnitude and the manner of failure in the target material will change as the distance from the jet axis increases. Distinct zones of failure are formed concentrically around the jet penetration axis: plastic zone, zone of small radial fractures, zone of large fractures. An understanding of the origin of the fracture zones and their time of formation in relation to detonation is necessary before shaped charges can be more efficiently used for well completion. The liner angle in the charge and the material of which the liner is constructed has been shown to influence the depth of jet penetration. Little has been published about the fracture penetration. Little has been published about the fracture phenomena in brittle targets resulting from the impact of jets of phenomena in brittle targets resulting from the impact of jets of different compositions and shaped-charge liner angles. The results of this study indicate that fracture orientation and propagation are not only a function of the charge geometry but also propagation are not only a function of the charge geometry but also a function of some physical characteristics of the target material. Introduction The May, 1888 issue of "Scribners Magazine" contained an article written by Charles E. Monroe describing his experiments with gun cotton. His experiment proved that the energy of an explosive could be concentrated by changing the geometry of an explosive charge. Cavities carved into gun cotton in the form of letters of the alphabet resulted, after detonation, in the mirror image of these letters being sunken into steel blocks. This cavity effect is known in the United States and England as the Monroe effect. The lined cavity effect was discovered by accident in 1936 by R. W. Woods at the Physics Dept. of John Hopkins University. During the investigation of an accidental death caused by a blasting cap, Woods found that the dimple in the base of the cap extruded a jet of high velocity copper particles.