Production analysis models are normally developed with the assumption of fixed drainage boundaries. However, this assumption is not always applicable. Examples where drainage areas may vary include: drilling of infill wells; shut-in of non-economic wells, and severe rate changes in some wells in the field. In such conditions, drainage volume of each well may not remain constant as time elapses.
A solution to this problem is simultaneous production analysis of all active wells in the reservoir to estimate total field gas-in- place instead of gas-in-place affected by individual wells. Such analysis requires a new boundary-dominated flow equation (BDFE) that forms a basis for analysis of field production data. In this study, a total normalized pseudopressure is defined. We show that a plot of total normalized pseudopressure versus real time forms a straight line with the slope equal to the reciprocal of total gas-in-place multiplied by initial gas compressibility. We further propose use of a new material balance time, referred to as total material balance pseudotime. We illustrate that a plot of single-well normalized pseudopressure versus this material balance time would result in a straight line with same slope. These plotting techniques account for conditions such as drilling new wells, closing damaged wells, and variations in well rate/pressure data. The introduced BDFEs are validated using several synthetic examples.
Two simulation cases are used to test the suggested techniques. In the first example, 4 wells are simultaneously producing with variable and different rates and in the second example, a new well is put on production in the reservoir every few months to simulate an infill program. In this example, 9 wells are drilled over a 6 year period. In another example, real field production data but with arbitrary reservoir systems also were used. These examples reveal the ability of the introduced techniques for estimating field gas-in-place and average pressure in multi-well gas systems. The method is very simple and can be extended to other unconventional reservoirs such as coalbed methane reservoirs and also reservoirs under two phase flow.