The Anderson "L" -An Abnormally Pressured Gas Reservoir in South Texas
Authors: Duggan Jack O. 
Published: December 2007

The presence of free liquid hydrocarbons below the dew point or of free water, even in small amounts, can cause flow problems in abnormally pressured wells with low permeability. However, many of the problems can pressured wells with low permeability. However, many of the problems can be solved with a little special handling, an understanding of how gas flows through porous media and how it behaves with changing pressures, and particularly with a good measure of engineering common sense. particularly with a good measure of engineering common sense.

Location and Discovery
The Mobil-David field was discovered by the Mobil Oil Corp. in May, 1965, in an area of unrelated shallow production. The field is located on the Lower Gulf Coast in Nueces County about 12 miles southwest of Corpus Christi, Tex. The Anderson "L" reservoir was discovered in Dec., 1965, by a confirmation well located 1.76 miles north of the discovery well of another field. The discovery of the Mobil-David field is credited to the subsurface mapping of the regional well data and to the theory of migrating structures with depth. A block of options and leases was obtained in an area of favorable sand buildup, and a Common Depth Point (CDP) seismic program was used to detail the Point (CDP) seismic program was used to detail the prospect and to locate the optimum wildcat location. prospect and to locate the optimum wildcat location. The new field discovery generated the drilling of 41 deep wells totaling 515,000 ft of hole and 11 shallower wells totaling 70,000 ft of hole. Twenty-four of the deep wells were completed in either the Middle Frio horizon or one of the two Lower Frio producing horizons, and 17 were dry holes. Twelve single-zone wells and one dual-lone well are currently on production. Almost every well found new faults that revealed the complexity of the deep structure. The practice of obtaining the current bottom-hole pressure in all nearby wells as each new well was completed, the use of pressure buildup tests and fluid analyses, and the use of continuous dipmeter logs in a well balanced logging program have been valuable in determining the faultblock separation between wells.

Geology
The field's two Lower Fro formation reservoirs of Oligocene age occur on a complexly faulted anticline, the axis of which trends NNE-SSW. The structure is 5 miles long and 3 1/2 miles wide. Commercial gas accumulations occur in the Anderson sand at 11,100 ft and in the Howell Height sand at 12,500 ft. The Anderson sand is a gray, fine-grained, firm sand. It is limy and slightly shaly. Log calculations and sidewall cores indicate a porosity of 24 percent and 0.1 md to more than 100 md permeability. Pressure buildup data indicate an effective permeability of about 8 md. Production from the Anderson sand has been found to occur on the upthrown side of the numerous down-to-the-southeast faults that are present on the northwest flank of the anticline. This paper will be concerned with one fault block referred to as the Anderson "L" reservoir. The Anderson "L" reservoir is separated from the other reservoirs by faulting and the separation is confirmed by markedly different fluid contents and pressure-production histories of the surrounding Anderson sand fault blocks. The Anderson "L" reservoir is depicted by the structure and isopachous map and the two cross-sections in Figs. 1 through 3. The isopachous map 2 band on a total-field gross-sand map. A factor of 75 to 80 percent should be used to estimate the net contributing reservoir sand. Characteristics typical of abnormally pressured sections are present.
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