Reservoir Heterogeneity and Permeability Barriers in the Vicksburg S Reservoir, McAllen Ranch Gas Field, Hidalgo County, Texas

This report documents research done on the mature and prolific McAllen Ranch gas field in South Texas, as part of the Secondary Gas Recovery (SGR) project funded by the Gas Research Institute, U.S. Department of Energy, and State of Texas.ABSTRACTA Secondary Gas Recovery project study in South Texas demonstrated that reservoir compartmentalization can result in reserve growth in a mature gas field. McAllen Ranch is an important field in that it typifies South Texas gas fields that produce from the deep, overpressured sandstone reservoirs of the Oligocene Vicksburg Formation. In this study, using the Vicksburg S reservoir in McAllen Ranch field as an example, we described heterogeneities that may compartmentalize lower Vicksburg gas reservoirs. A numerical model of part of the Vicksburg S reservoir demonstrates that pressure differences between adjacent wells result from low-permeability barriers to gas flow within the reservoir, not from the generally low permeability of the reservoir; therefore, pressure differences are partially the result of compartmentalization.McAllen Ranch field produces from 33 lower Vicksburg reservoirs at depths that range from 9,000 to 14,000 ft (3,000 to 4,260 m). Our research concentrated on the Vicksburg S reservoir, which consists of six stacked progradational sandstones termed S1 through S6. This reservoir is the most productive in the field, having had a cumulative production of 162 Bcf by the end of 1988.Faults, depositional heterogeneity, and diagenetic heterogeneity are all important agents of compartmentalization in the lower Vicksburg reservoirs. In the Vicksburg S reservoir, each type of heterogeneity may be important in different areas of McAllen Ranch field. In south McAllen Ranch field, the Vicksburg S reservoir is compartmentalized by faults. Distributary-channel-fill sandstones are the most likely to contain untapped reserves because they are laterally discontinuous. In the north, or B area, the S reservoir contains thick, laterally continuous delta-front sandstones and offers the poorest potential for depositional compartments. Diffuse, diagenetically created zones of low permeability are probably the most important restrictions to gas flow in the B area.Depositional heterogeneity that may inhibit the flow of natural gas is present in McAllen Ranch reservoirs on four different scales: (1) The largest scale of heterogeneity is that of individual small and strike-elongate deltas (-3.5 x 7 mi [-5.5 x 11 km]). (2) Within each delta are lobes 1,000 to 4,000 ft (300 to 1,200 m) in width. (3) Individual distributary-channel bodies are 3,000 to 9,000 ft (1,000 to 3,000 m) in width. (4) Within individual sandstone bodies, higher permeability zones that are 1 to 2 ft (0.3 to 0.6 m) in thickness are in the coarsest distributary-channel and delta-front sandstones. Three different diagenetic facies are present in McAllen Ranch cores: quartz cemented, calcite cemented, and chlorite cemented. Intercalation of the three diagenetic facies results in heterogeneous porosity and permeability distributions. Diagenetically controlled low-permeability zones are probably the most important factors in creating permeability barriers in north McAllen Ranch field, where sandstone bodies are laterally continuous.Original reservoir pressures were measured in wells drilled within the developed parts of the field long after initial production. The newly completed wells are adjacent to older wells that have shut-in pressures as much as 8,000 psi lower, suggesting that heterogeneities within the reservoirs may create barriers to gas flow between the newer and older wells. Analysis of production and pressure data indicates that neither water drive nor compaction is an important mechanism in the Vicksburg S reservoir. We created a finite-element model of the S4 sandstone, demonstrating that barriers to gas flow must exist between wells and that high pressures and gas production in the new wells result from compartmentalization of the reservoir rather than from low permeability.The presence of permeability barriers does not necessarily indicate reservoir compartmentalization that results in increased reserves of natural gas. Reserve increases from lower Vicksburg reservoirs may form a significant resource when the original completion spacing is greater than the compartment sizes. Assessment of reservoir compartmentalization is difficult in low-permeability reservoirs. Nevertheless, in this part of McAllen Ranch field, the field operator, guided in part by a 3-D seismic survey, has indicated a reserve growth potential of more than 100 Bcf. This reserve growth is due to reinterpretation of reservoir distributions in relation to structure and not to reservoir compartmentalization within a single reservoir. However, reservoir compartmentalization has resulted in a minimum of 1.12 Bcf of reserve growth from two S2 recompletions out of six attempts in the S2 and S4 sandstones