A Systematic Approach to Resolving Wellbores Stability Issues While Drilling through Depleted Sandstone Reservoirs, Case Study-GAB Field, Niger Delta

Abstract

The study area is characterized by sequences of sandstone and shale formations. Hydrocarbon production depletes the pore pressures within the sandstone reservoirs while the shale formations retain their original pressures. This leads to the narrowing of the safe mud weight window while drilling and increases the probability of the occurrence of wellbore stability issues such as loss circulation, tight spots, stuck pipe and hole collapse during drilling and casing run activities. Depleted reservoirs were traversed while drilling through the intermediate (12-1/4") hole section in GAB-7H. It was drilled with 9.0ppg equivalent mud weight (EMW) and an equivalent circulating density (ECD) of 9.6ppg EMW to the target depth. While running the 9 5/8" casing, it was observed that the wellbore had collapsed, thereby preventing the casing from getting to bottom of the hole which led to the abandonment of the hole section and a consequent side-track. This paper presents the lesson learnt and best practice that were adopted for GAB-7Hst and subsequent wells in the GAB field. Prior to the drilling of the sidetrack, a one-dimensional mechanical earth model (MEM) was constructed using petrophysical logs and formation tests of GAB-7H and other offset wells. Shale pore pressure was derived from gamma-ray, resistivity and sonic logs using the Eaton's and Bower's methods while sand pressures were measured/ estimated from modular dynamic testers (MDTs) and depletion models. The fracture gradient was derived using Matthew's and Kelly equation. Shear failure gradient was calculated using Modified Lade equations and log derived mechanical rock properties. The post-drill analysis of the offset wells was then calibrated with the drilling events and mud weights used. This revealed that the mud weight used to drill the 12-1/4" in GAB-7H was inadequate. An optimum mud weight program coupled with close monitoring of ECD is a key requirement to successful well construction in the GAB field, where several reservoirs at various states of depletion, sandwiched by shale formations are traversed. These has led to several successful drilling operations in the field.