Gas reservoirs producing under pressure support sometimes exhibit a nonlinear relationship between cumulative production and (P/Z). A corollary effect is that the apparent gas volume in place increases with time and exhibits a sensitivity to changes in withdrawal rate. The nonlinear behavior may be attributed to dual porosity effects or aquifer support, which are both accounted for in the model. Without an aquifer being present, the nonlinear behavior may be a result of adsorption, natural fractures or an extremely tight gas-bearing region adjacent to a main reservoir. Proper modelling of such a system is crucial in determining ultimate recovery, and in choosing an appropriate development scheme.
In this paper we present results of a material balance model developed to study a dolomite, dual porosity gas reservoir in Alberta. The study was done to explain the better than expected performance of the reservoir, and to forecast future reservoir performance. The material balance model consisted of a main 'productive' tank in pressure communication with a low permeability, gas bearing matrix. Gas influx from the outer low permeability matrix results from pressure drop in the reservoir, and provides pressure and material support. Tubing, compressor and pipeline effects were analytically modelled and coupled with the material balance and deliverability models.
A single coupled analytical equation has been developed to account for reservoir, wellbore and surface facilities (compression, surface line) simultaneously.
An excellent history match was obtained with the model by properly adjusting the sizes and properties of the reservoir and matrix. Long-term performance was predicted under various schemes and constraints, including the effects of additional wells and compression. The results show that gas influx from matrix and ultimate recovery are very sensitive to withdrawal rate, and late-time performance of the system is observed to be influx-dominated. The impact of these results on future operating decisions is shown to be significant and is discussed in detail in the paper.
Many tight gas reservoirs are known to sustain low production rates for a considerable length of time. This may be due to the fact that these reservoirs are characterized by two regions of very different permeability. This difference may be related to depositional factors or may be due to the existence of natural fractures.