Mitigate Drilling Risks in Highly Depleted Reservoir Fields by IntelligentlyPlanning Well Trajectory for a CO2 Sequestration Project

摘要

A highly depleted gas field in the southern North Sea has been selected as a potential CO2 storage site. Data from drilling ofproduction wells shows that wells experienced substantial tight spots, overpulls, cavings, mud losses, even pack offs, andconsiderable time was spent in reaming tight holes and circulating excessive cuttings. To mitigate the drilling risks for new CO2injection wellbores, a geomechanical study was undertaken to understand the drilling issues and to quantify a practical mud-weightwindow. The study used offset well drilling and wireline log data to derive field stresses, formation pressure, rock strength and elasticproperties. A practical workflow was developed to characterise the interaction between pressure depletion and fracture gradientchanges. In this particular case the results showed that the fracture gradient was as low as 9.3 ppg, and the wellbore collapsepressure in the overburden shale was highly dependent on the well trajectory. A vertical well could be drilled safely with ahydrostatic mud weight of ~8.6 ppg. A wellbore deviating at more than 65o would require mud weight approaching 9.3 ppg, thefracture gradient, to prevent wellbore failure. This leads to a tight mud-weight window for drilling management. If an operationmud-weight window of 0.5 ppg is required, the well inclination should be below 65o if it is planned towards the minimumhorizontal stress Shmin orientation, or less than 45o if towards the maximum horizontal stress Shmax azimuth to mitigate drilling risks. This paper describes a geomechananical workflow that assists in understanding the interaction between pressure depletion andfracture gradient changes from which it is possible to quantify a mud-weight window for safe drilling. The workflow descriptionshows how the approach can help mitigate drilling risks and minimise drilling costs by optimising well trajectories and thereforeplay a significant role in the well planning and drilling management of depleted reservoirs.