Publications

Tunu is a giant gas field located in the Mahakam Delta, Indonesia. More than 1000 wells have been drilled since 1991 to produce multi-layer sandstone reservoirs deposited in a deltaic environment, dominated by distributary channels and mouth bars. The reservoirs are depleted to a variable extent. Drilling through highly depleted and overpressure reservoirs in a single well has multiple challenges, such as mud losses, differential sticking, difficult logging and the inability to reach the target. These problems can significantly increase non-productive drilling time and total well cost. Fracture pressure prediction is important in designing casing and mud programs which are parameters to avoid mud losses while drilling. In the last few years, several mud losses events have occurred in Tunu Field wells despite drilling with equivalent circulating density value lower than leak of test. A relationship between the depleted reservoir with the losses events is being suspected. Further suspicion is raised for the fracture pressure value of strong depleted reservoir giving rise to the weakest point being no longer directly below the casing shoe. Fracture pressure calculation is a function of independent variables, viz. pore pressure, overburden pressure and the poisson’s ratio. Normally, the former two parameters are obtained from formation pressure tests and density logging, and the latter from LOT data of shale formation below the casing shoe. In this study however, the poisson’s ratio in the depleted reservoir is estimated from mini frac, step rate test and equivalent circulating density from mud losses events. The adapted method has been tested for multiple recent drilled wells and the results show mud losses are related to decreased fracture pressure within the depleted reservoir. This fracture pressure estimation method can give additional consideration to predict well fracture pressure in depleted reservoirs in order to prevent drilling mud losses.