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Collaborating Authors
Results
Abstract Coal is not an inert reservoir rock and reacts to gas desorbed from its surface. Coal matrix shrinks as gas is desorbed, increasing cleat width and, therefore, permeability. Very few coal matrix shrinkage data have been reported in the literature so a series of experiments was undertaken to measure such data at reservoir pressures, temperatures, and 100% relative humidity. Strain gages were affixed to the coal sample in the face and butt cleat directions as well as the vertical direction. This work reports measured deformations of a sample of high volatile C bituminous coal from the San Jan Basin during sorption and desorption of first methane then CO2. A pressure cycle was also run with helium, a nonsorbing gas, to determine mechanical compliance of the sample. Observed strain gage behaviors are discussed and shrinkage coefficients for both gases reported. Matrix shrinkage was found to correlate with gas content rather than pressure, confirming the work of a previous investigator. Shrinkage coefficients varied more among replicate gages aligned in the same direction than between gages in different directions. Anisotropic shrinkage effects are discussed. Using a matchstick geometry model, equations are derived for permeability change due to matrix shrinkage. Coefficients reported here are used in example calculations of absolute permeability and porosity increases during coalbed depletion. Introduction Coal matrix swells and shrinks as gas is first adsorbed then desorbed. The amount of swelling depends on coal rank and sorbed gas composition. During recovery of coalbed methane, coal matrix will shrink, increasing cleat width. As permeability depends on the cube of cleat width, a small increase in cleat width may significantly increase permeability. Such an increase will offset permeability losses due to increased stress during depletion. A search of the literature revealed eight previous studies of coal matrix shrinkage.14 These studies are summarized in Table I. The matrix shrinkage coefficients ranged from a low of 8.62E-7 psi-1 to a high of 6.55E-4 psi-1. Moffat and Weale investigated sorption induced swelling of low volatile bituminous and semi-anthracite coals. They determined a methane swell in coefficient of 1.70E-6 psi-1 at a pressure of 200 atm. Gun the investigated swelling due to methane and carbon dioxide on coals with ranks ranging from high volatile A to anthracite. Reported swelling coefficient ranged from 2.76 to 6.90E-6 psi-1 and carbon dioxide was reported to swell coal more than did methane. Vinokurova found low rank coals swelled more than high rank coals but no swelling coefficients were reported. Wubben, et al., investigated swelling of anthracite and bituminous coals and reported swelling coefficients of 1.4 to 6.9E-6 psi-1. Reucroft and Patel used coals from the Appalachian Basin of the USA to investigate swelling due to sorption of carbon dioxide, nitrogen, and xenon. They reported a carbon dioxide swelling coefficient of 6.55E-6 psi-1. Gray measured a methane swelling coefficient of 8.62E-7 psi-1 for a Japanese coal of unreported rank. No details of coal rank were provided by Juntgen when he reported a methane swelling coefficient of 2.57E-4 psi-1. Only one of the previous studies, Harpalani and Schraufnagel, dealt with a coal currently of interest to coalbed methane recovery. Using a sample of Piceance Basin coal, they reported a shrinkage coefficient of 6.2E-6 psi-1. Unfortunately, many of these studies were done at pressures below those typically encountered in CBM fields and none of them used reservoir temperatures or the high humidity conditions typical of in situ coals. Nor did any provide information on ash content (or mineral matter) which would intuitively have a strong influence on swelling behavior as it significantly affects the amount of gas which can be sorbed on a sample. Experimental Objectives of our work were to assemble experimental apparatus, develop experimental procedures, and then measure coal matrix shrinkage coefficients at reservoir conditions. Reservoir temperatures and pressures commonly encountered during coalbed methane recovery are not extreme conditions for strain gages. Many commercial gages would probably be quite adequate for this experiment. P. 575
- North America > United States > Colorado (0.48)
- North America > United States > West Virginia (0.34)
- North America > United States > Pennsylvania (0.34)
- (2 more...)
- North America > United States > New Mexico > San Juan Basin (0.99)
- North America > United States > Colorado > San Juan Basin (0.99)
- North America > United States > Colorado > Piceance Basin > Williams Fork Formation (0.99)
- North America > United States > Arizona > San Juan Basin (0.99)
Completions and Stimulations for Coalbed Methane Wells
Palmer, Ian (Amoco) | Vaziri, Hans (Technical University of Nova Scotia) | Khodaverdian, Mohamad (TerraTek) | McLennan, John (TerraTek) | Prasad, K.V.K. (Amoco) | Edwards, Paul (Amoco) | Brackin, Courtney (Amoco) | Kutas, Mike (Amoco) | Fincher, Rhon (Amoco)
Ian Palmer,* Amoco, Hans Vaziri, Technical University of Nova Scotia, Mohamad Khodaverdian,* Terra Tek, John McLennan,* TerraTek, K. V. K. Prasad,* Amoco, Paul Edwards,* Amoco, Courtney Brackin, Amoco, Mike Kutas, Amoco, Rhon Fincher, Amoco Abstract Amoco is producing coalbed methane from several hundred wells in both San Juan and Warrior basins. These wells were completed/stimulated in one of two ways:openhole cavIty completions. hydraulic fracture stimulations through perforations in casing. cavity operations are described, and new data from several cavity completions is presented and analyzed. The latest geomechanics modeling of the formation of cavities in coalbeds is presented. The model allows the growth of a cavity as tensile failure occurs, and computes increases in permeability in a stress-relief zone that extends tens of feet from the well. critical parameters are given for the success of cavity completions. A pulse interference analysis is discussed: as well as interwell permeability, this can provide information on stress-dependent permeability. Finally, some wells which were originally cavitated did not perform up to expectation, and have been recavitated with remarkable success - these are also examined. Amoco has tried several different kinds of hydraulic fracturing treatments. Results of comparisons between foam fracture, slick water fracture, and gel fracture treatments are presented. Statistical comparisons are given for regions outside of the fairway zone in the San Juan Basin. In the Warrior Basin, water fracture treatments with and without sand have been compared. Lastly, foamed water cleanouts, without sand, have been deployed, and their success is reviewed. Introduction In this paper we present new information on completions/stimulations of coalbed methane wells. Specifically. we discuss (1) openhole cavity completions in the fairway (sweet spot) of the San Juan Basin (Colorado and New Mexico - see Figure 1), and (2) fracture stimulations in the San Juan Basin and the Warrior Basin (Alabama). Cavity Operations The openhole cavity completion has been used with tremendous success in the San Juan Basin. Some wells produce in excess of 10 MMCFD from only 3,000 ft depth in the fairway zone (Figure 1). In the cavity operation, a series of injections (or shut-ins) and blowdowns (actually, a controlled blowout) is performed over typically a two-week period. Coal fails and sloughs into the wellbore and is ejected from the well, leading to creation of a cavity (enlarged wellbore). A plastic or shear failure zone is also formed beyond the cavity. and in this region the permeability is changed. A typical Amoco cavity operation was described previously. Below is an elaboration of certain aspects of cavity operations in the San Juan Basin fairway:The openhole portion of the well is generally 200โ300 ft in height, containing usually more than 50 ft of net coal. The coals are divided into the basal coals, which are usually the more productive, and the upper coals. Normally 7-in. casing is topset above the top coal, and TD is only a couple feet below the bottom coal. A typical cavity operation entails a sequence of (1) cleanout of the well in the evening using air (1,500โ2,200 SCFM) and water (20โ100 BPH) injections, followed by (2) flow testing lasting typically four hours, followed by (3) cavity operations (or CST), typically 6-10 surges during the daytime. Before the flow test and CST, the bit is either pulled into the casing shoe or to the surface. The sequence is repeated many times over typically 10โ20 days. All flow tests are conducted through a 3/4 in. choke, typically for four hours. All pressure surgings are conducted by rapidly opening a surface valve, allowing gas and water and coal fines to be expelled through blooie lines to the pit. The basal seams seem to respond more than the upper seams to the cavity operations, presumably because they are more friable. It is not uncommon to see 0.5โ1 in. pieces of coal come to the surface during cavity operations. In flow tests in good wells, flows during the cavity operations often decrease with time over 1-4 hrs. This may be the transient effect that is predicted by the cavity modeling (see later in this paper). P. 583
- North America > United States > New Mexico (1.00)
- North America > United States > Colorado (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (0.88)
- North America > United States > Oklahoma > Arkoma Basin > Cana Woodford Shale Formation (0.99)
- North America > United States > New Mexico > San Juan Basin (0.99)
- North America > United States > Colorado > San Juan Basin (0.99)
- (4 more...)
Abstract Economic gas production from coal seams (in which cavitation is not applicable) requires hydraulic fracture stimulation due to the low reservoir permeability of most target coal seams. Effective hydraulic fracture stimulation of coal seams, however, presents a formidable challenge due to a number of factors including: the mechanical complexity of coal; prevalent natural fracturing; extreme sensitivity of coal seams to fracturing fluid damage; stress sensitive permeability of coal; and the (often) complex geometry of the induced hydraulic fractures. The authors began by performing an in-depth analysis of previous hydraulic fracturing treatments performed in coal seam reservoirs in China. The results of this post-treatment analysis, together with the collected reservoir data, led to the development of a modified stimulation strategy. As part of a pilot investigation by the North China Bureau of Petroleum Geology (NCBPG) of a (potentially) large coalbed methane field in China, eight hydraulic fracturing treatments were executed in the summer of 1994 using the modified stimulation strategyโ including on-site real- data three-dimensional hydraulic fracture modeling and treatment re-design. This paper describes the analysis theory and methodology, the novel stimulation techniques employed, the on-site implementation (including difficulties), and the results of the fracturing treatments. While the paper is a case study of fracturing at a particular field, the paper will also contain a discussion of some of the complexities of coal seam fracturing. The issues to be discussed are: simultaneous propagation of multiple hydraulic fractures; (often) excessive near-wellbore fracture tortuosity; rapid downward proppant convection; and the high net fracturing pressures which may result in exceeding more than one of the reservoir's principle stresses. Analysis of the collected data shows that 2-D models and "conventional" 3-D models of the hydraulic fracturing process apply very poorly to hydraulic fracturing in coal seams. Engineering decisions based on these more "conventional" fracture modeling techniques can lead to inappropriate fracture treatment design, as well as significant problems in predicting the post-frac production performance. P. 371
- Asia > China (1.00)
- North America > United States > California (0.46)
- North America > United States > Alabama (0.30)
- North America > United States > Texas (0.28)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > Asia Government > China Government (0.34)
- North America > United States > New Mexico > San Juan Basin (0.99)
- North America > United States > Colorado > San Juan Basin (0.99)
- North America > United States > California > San Joaquin Basin > Lost Hills Field (0.99)
- (7 more...)
Abstract Coalbed methane is an abundant resource in the U.S.A., for example, in the San Juan Basin (50 TCF), and Black Warrior Basin (20 TCF). Amoco has drilled many wells in both basins in the last few years. Different completions and stimulations have been tried, and these are summarized as follows:Openhole cavity - This has worked best in areas of the San Juan Basin where reservoir pressure and permeability are high. The wells can be prolific producers (up to 12 MMCFD). The physical mechanisms involved in the completion are discussed, and these are used to try to understand the difference in gas production between cavity completions and gel fracture stimulations. Gel fracture treatments - These stimulations are conducted through perforations in coal seams, and sometimes two or more stages are used per well. High fracture conductivities are achieved by using 12/20 mesh sand to concentrations of 10 ppg, and the viscous gel should mean most of the coal seams are propped. Graduated proppant treatments (40/70 sand preceding the main 12/20sand) are often used to alleviate screen-outs. Although some gel damage to the coal formation is evident, moderate productivity increases have been achieved. Water fracture treatments Because of gel damage to the formation, fracturing treatments have been conducted using water as fracture fluid, plus 12/20 sand to concentrations of a few ppg. Although not all coal seams are expected to be propped (due to low viscosity of water), gas production is greater in general than offset wells with gel fracture treatments, and the water fractures are cheaper. Sandless water fracture treatments - Recently water fracture treatments have been performed without sand, using ball sealers to open up more seams. Although their gas production may not be as good as wells fractured with water and sand, they can be substantially cheaper. Fracture designs are summarized, and gas production is compared for the different completion/stimulation techniques. Fracture treatments in general appear to fall into two different classes: First, those in which significant fracture height growth occurs, accompanied by falling pressures (vertical fractures). The second class are pressures (vertical fractures). The second class are those which are largely confined by the coal seams, and are accompanied by steady or rising pressures, proppant-induced pressure increases, and ISIP values greater proppant-induced pressure increases, and ISIP values greater than 1 psi/ft (T-fractures). Interpretations of fracture geometry, height growth or confinement, fracturing pressure, and proppant-induced behavior, are pressure, and proppant-induced behavior, are summarized. Introduction Hydraulic fracturing of coalbeds, which is the most common form of completion/stimulation, is not fundamentally different from fracturing of conventional formations, although some adaptations have certainly been made. However, other completion/stimulation techniques such as the openhole cavity have been reintroduced recently, and when they have worked they appear to be more productive than the fracture stimulations. Our understanding of these completions/stimulations is still evolving. P. 679
- North America > United States > New Mexico > San Juan County (0.92)
- North America > United States > Alabama > Tuscaloosa County (0.92)
- North America > United States > New Mexico > San Juan Basin > Fulcher Kutz Field (0.99)
- North America > United States > New Mexico > San Juan Basin > Cedar Hill Field (0.99)
- North America > United States > New Mexico > San Juan Basin > Blanco Field (0.99)
- (12 more...)