Challenges of Drilling Deep Wells in a Complex Overburden with Severe Depletion and Experiences From Caspian Sea

Abstract

AbstractThe objective of this paper is to share Dragon Oil's experience to overcome challenges in drilling deep depleted reservoirs in the Cheleken Contract Area, Caspian Sea, Turkmenistan. Dragon Oil operates two producing oilfields; Lam and Zhdanov. The main targets described in this paper are from the Zhadanov field with the deepest target reaching around 4000m. This paper describes the challenges and experience from drilling complex wells in this area.The wells described in this paper penetrate various depleted reservoirs due to past production. The deepest reservoir consists of a sequence of interbedded sands with varying pore pressure due to geology and depletion due to production. The intra-reservoir shale layers are unstable and require high Mud Weights, often reaching technical limits (up to 20 ppg).The requirement of high mud weight to address shale instability is in contrast with the solutions for drilling depleted sand and the common drilling risks are mud losses and stuck pipe. An analysis of challenges, risk mitigations and lessons learned are discussed in this paper.Drilling a deep 6" directional hole is challenging and it requires continuous monitoring of the drilling fluid density. The pre-drill well planning is a key solution for preparing to deal with operational challenges and develop a strategy for reducing risk. This paper describes the important steps followed in the well planning process, risk mitigation options analyzed and incorporating the key elements in drilling program to overcome challenges.Some of the lessons learned from the experience of delivering wells in such complex area are: Implement 3D/4D PPFG and wellbore stability models to communicate depletion magnitudes accurately and effectively incorporate the risk in well planning to enable drilling with minimum overbalance.Map the correlation between depleted sand bodies (e.g. facies model) and assess impact of fault typesImplement Synthetic Oil Based Mud to improve wellbore condition and enhance drilling performance.Choose well trajectories to help transfer weight/torque to bottomMinimize the BHA static time across severely depleted zonesThis paper shares the experience from Turkmenistan and lessons learned from Dragon Oil's operations in Caspian Sea. This paper consolidates the well planning process, methods adopted to incorporate risk mitigation into drilling program and our recent experience from drilling complex wells targeting deep depleted reservoirs.