Adverse Influence of Stratification on a Gas Cycling Project
Authors: Sprinkle T.L. , Merrick R.J. , Caudle B.H. 
Published: December 2007

Described here is a gas cycling project that failed. The premature dry-gas breakthrough and subsequent poor recovery in this Gulf Coast field was probably caused by horizontal permeability stratification. probably caused by horizontal permeability stratification.

Introduction
The subject field in this study is a small fault-block gas-condensate reservoir in Galveston County, Tex. The reservoir is in the Frio sand and has an average net pay thickness of 64 ft. The displaceable porosity for gas cycling (the total porosity less the saturation of connote water) averaged 0.19 over this thickness. The reservoir is believed to be completely isolated by four sealing faults, as shown in Fig. 1. The surface area of the reservoir is given as 234 acres. These figures give a displaceable pore volume af 123.85 MMcf at the average reservoir conditions (320 atm pressure and 727 degrees R) at the start of the cycling project. This is equivalent to 29,352 MMscf of wet gas as the potential recovery, which would result in nearly 3 million bbl of condensate. The first well in the reservoir (Well 1) was the sole production well until it was converted to an injection production well until it was converted to an injection well upon completion of Well 2 in Nov., 1966. Prior to that time, 11,504 MMscf of wet gas was produced, and the reservoir pressure declined to near the dew point (320 atm). point (320 atm). Since no core information was available from the earlier well, a full 4-in. core was taken during the drilling of Well 2. Representative samples were analyzed by a commercial laboratory (Table 1). The remainder of the core was taken for the detailed analysis described in the next section. It was hoped that with such an analysis an accurate prediction of the cycling results could be obtained. The analysis was to pay particular attention to permeability variations, which particular attention to permeability variations, which would have a large effect on cycling results. Cycling was started on Jan. 20, 1967, with injection into Well 1 and production from Well 2. Over all, a volumetric replacement of the produced gas was closely achieved. Water production was relatively stable at 50 to 100 B/D.

Core Analysis
A complete description of the analysis procedure and results may be found in another publication. Briefly, those parts of the core that were suitable were subjected to a whole-core type of permeability measurement similar to a flood-pot analysis. These tests were designed to detect horizontal permeabilities of individual strata as small as 0.1 in. in height. The core samples were first formed into a right cylinder with concentric center hole. The ends were sealed so that flow would be between the center hole and the outer curved surface of the core. The cores were saturated with naphtha, and this was displaced from the inside by the same naphtha carrying a red dye and at a constant flow potential difference. The inner surfaces of the sandstone were pretreated with a surface active agent that prevented adsorption of the dye from solution, and the outer surfaces were dusted with a white clay so that the arrival of the red dye could be seen and photographed.
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