Case Study: Geomechanics Enables Successful Horizontal-Well Drilling in Libya
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 111384, "Geomechanics Enables the Success of Horizontal-Well Drilling in Libya: A Case Study," by Kaibin Qiu, SPE, Schlumberger; Julio Gonzalez Felgueroso, Gabino Lalinde, Abdulmagid Naas, and Bernard Coste, Akakus Oil Operations; and John Fuller, SPE, Schlumberger, prepared for the 2008 IADC/SPE Drilling Conference, Orlando, Florida, 4-6 March. The paper has not been peer reviewed.
Highly deviated or horizontal wells can incur instability problems. A case in Libya is presented in which significant difficulties were encountered during drilling of the first horizontal development well in a field in the Murzuq basin. The first two branches of the well were lost to severe instability problems. A comprehensive geomechanics study was carried out to determine the causes of the wellbore failure and to improve drilling design and drilling performance of additional development wells in the field.

Introduction
Horizontal wells can increase production rates and ultimate recovery and reduce the number of platforms or wells required to develop a reservoir. The geometry also helps to delay water or gas breakthrough, bypass environmentally sensitive areas, and reduce stimulation costs. In 2006, Akakus Oil Operations began drilling of the first horizontal development well, Well H1, in a field in Libya to avoid water coning and delay water breakthrough. However, unexpected drilling difficulties were encountered, and the first two branches of the well were lost. Before this project, many vertical exploration and development wells had been drilled in the same block without experiencing any major problem. A wellbore-stability study was carried out to determine the cause of wellbore failure in the horizontal well. A mechanical Earth model (MEM) was built for the analysis. The analysis identified the cause of wellbore instability as inadequate mud weight while drilling the overlying shale formation in the deviation-buildup section. The design of the second horizontal well was optimized on the basis of this study, and it was drilled without problems and ahead of drilling schedule. This case demonstrated that a comprehensive geomechanics analysis could improve drilling performance and reduce drilling costs.

Data Audit
Erroneous or inadequate input data impair wellbore-stability-analysis results, even in the most advanced analysis model. Therefore, a data audit is the critical first step. Relevant data for a geomechanical study were selected and gathered. Data included extensive information from different domains, such as daily drilling reports, master logs, wireline logs, geological data and reports, petrophysical interpretations, seismic data, and general field information. Those data were analyzed and cataloged, and data availability was identified. Redundancy of data and use of data from different sources and domains were highly desirable to achieve an internally consistent description of rock properties and stresses. Cross-checking and comparing data from alternative sources can highlight errors, inconsistencies, and omissions in the data that may need to be fixed or filled in at a later stage to improve the robustness of the predictions.