Pressure transient analysis (PTA) for a multi-layer well, producing commingled from more than two non-communicating reservoirs, has always been a challenge. To address this challenge, innovative data acquisition techniques and novel multi-layer analysis procedures were developed in the 1980's. However, very limited practical application of these theoretical procedures has been reported in the literature.
This paper attempts to fill in this important gap. The paper summarizes the practical application of different theoretical multilayer testing (MLT) data acquisition and procedures to commingled wells in the RasGas Company Limited (RasGas) operated area of Qatar's North Field, the largest offshore non-associated gas reservoir.
Comparison of the different MLT analysis methods as applied in RasGas suggests that the rate-normalized (RN) PTA method, despite its strong theoretical basis, is the least reliable in this field application. On the other hand, in spite of their theoretical limitations, results from the Selective Inflow Performance (SIP) method obtained during this work are more reliable and consistent with other observed data (cross-flow, well productivity, etc).
Based on the above work, a way forward in MLT analysis has been developed, relying on the SIP testing approach to provide not only information on changes in individual layer pressures, but also reliable time-lapse changes in individual layer productivity, which is very important for monitoring individual reservoirs in commingled wells.
The North Field, located offshore in Qatari waters, represents the largest accumulation of non-associated gas in the world. RasGas Company Limited is one of the major operators of the North Field in Qatar, producing since 1999.