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

摘要

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