Attempting To Quantify Subsurface Uncertainties With Dynamic Data – A Reservoir Management Case Study

Abstract

Abstract Analyses of dynamic data that include pressure and production history have long been recognized as a tool to evaluate the underground reservoir size, fluid volumes and future performance. This study encompasses a similar case utilizing analytical and numerical methods to interpret the dynamic data. As a result several geological features were confirmed and a few sub-seismic features were identified. The field under discussion is located in Lower Indus Basin of Pakistan and characterized by hydrocarbon accumulations in Lower-Goru Upper-Sand reservoirs. Exploratory well A-1 discovered the field by finding gas bearing Sandstone reservoir established based on formation evaluation and open-hole log profile. The structure was bounded by two major intersecting faults and one splay fault, all juxtaposed against shale barriers. Due to these structural features A-Sand was expected to exhibit depletion drive mechanism. When A-1 was put on production, it performed as a dry gas producer for brief period of time then its gas rates started declining with increasing CGR. Routine surveillance recorded depletion in reservoir pressure and the well loaded up after a year. After reviving on Gas Lift and A-1 became essentially an oil producer with a GOR of 2000 scf/stb. BHP surveys showed no further depletion than 400 psia. These facts hinted at pressure support to A Sand reservoir as opposed to originally assumed closed structure. To enhance oil production hydraulic jet pump was installed and achieved apex oil rates from the well, interestingly, without commensurate increase in water production. On Jet Pump also, well exhibited fairly constant liquid withdrawals strengthened idea of pressure recharging. To identify source of A-Sand recharging, it was decided to (1) analyse pressure-production history (2) Closely analyse interpreted analytical model built on pressure data (3) Perform numerical PTA by integrating G&G data, and (4) develop Allen diagrams to see possible juxtaposition with downthrown blocks. The study concluded that Splay fault on reservoir structure may not be completely sealing since both Analytical and Numerical Model transient models strongly suggested slightly different position of splay fault on the west of the well. Allen Diagram further showed decreasing throw of the same fault intimating possibility of juxtaposition. Hence, it was concluded that the oil source and pressure support most probably lies beyond the western fault and could be confirmed by refining subsurface structure through re-interpretation of seismic data.This way, the work emphasized the role of dynamic data in adding value to a company's knowledge of subsurface elements and hence widen the scope of field development strategy.