In stacked reservoirs with limited accumulation, it's a practice to produce reservoirs commingled, but there are always challenges in the allocation of production and estimation of in-place for such reservoirs. This paper presents an analytical approach using Material Balance (MBALTM) reservoir allocation tool for allocating production and inplace estimates on individual basis for such reservoirs.

MBALTM allocation tool, unlike the traditional allocation approach of using product of (kh), works in a twostep process; (i) Defining inflow for an individual well at each time step and (ii) Setting up Material Balance model that accounts for the rate of depletion which will correct inflows at each time step. This study highlights the approach of MBal reservoir allocation tool on a gas field with significant water production, producing from two different units of Sand-1. In this regard, determination of GIIP and aquifer strength was a great challenge. Production logging data was acquired which showed significant gas and water production from bottom unit of Sand-1. Bottom hole flowing pressure (BHFP) and individual zonal contribution in terms of rate acquired from production logging data was used as a history matching parameter. Regression of darcy and non-darcy parameters aids in matching production logging results, which in turn develops inflow performance for an individual layer at each time step. Apart from this, measured static bottom-hole pressure (SBHP) was used in order to determine the impact of depletion at each time step, further supplementing development of inflow performance.

Finally, production data on individual reservoir level was determined based on production logging data calibration, supplemented by static pressures matched using gas and aquifer volume. Matched results estimated Sand-1 Unit-1 GIIP to be around 74 Bcf with depletion drive trend, whereas bottom unit exhibited weak to moderate water drive behavior with GIIP of around 19 Bcf. Based on these results, bottom unit wasn't isolated and left open to drain remaining potential, thereby outrunning the aquifer in order to maximize its recovery.

In scenarios, where we are producing from multiple reservoirs, production from reservoir with inferior quality is masked by prolific reservoir, whereas on other side reservoirs with significant water production masks reservoir with water free production. It is necessary to plan development and depletion strategies for such reservoirs, but prior that price in terms of potential needs to be addressed. Using this technique, one can overcome challenges in multiple stacked reservoirs.

You can access this article if you purchase or spend a download.