The Opuama field is in the northern part of the Niger Delta, 92 km northwest of Warri. It has an area of about 6 km2 and comprises of a rollover anticline developed between two northwest/southeast-trending listric growth faults. To date, liquid hydrocarbons have been encountered in eight reservoirs of Oligocene age. Five wells had been drilled by 2006 when the wells were shut in. In 2016, production started again in this field.
A new drilling campaign has been planned for exploratory wells. The first of these wells (designated as Well-C), is planned to be the first deviated well to be drilled in this field to produce from deep reservoirs. The reservoirs in question are clastics representing a deltaic depositional environment and consisting of a thick coarsening-up sand sequence. Previous wells drilled in the field were vertical to sub-vertical, and drilling issues such as tight hole, kicks, and stuck pipe were encountered regularly. This led to poor logging conditions in which only very sparse and poor-quality data were acquired. The objective of the new drilling campaign is to determine what best practices are needed to enable optimized drilling by minimizing wellbore instability issues in the planned well C and for future deviated wells to be drilled in this field. A geomechanics-based approach integrating acoustic measurements, basic logs, and drilling events was adopted to provide a solution to reduce drilling-related problems and to ensure improved drilling rates. Two offset wells, Well-A and Well-B, situated in the same fault block, were seen upon exhaustive data auditing to contain the most comprehensive amount of data of any of the wells available. These were selected as offset wells for this analysis. Based on the post-drill analysis of the offset wells, a predrill wellbore stability model was generated for planned Well-A.
The post-drill analysis showed a pore pressure ramp in the deep reservoirs in Well-B, which was the probable reason for kicks encountered in that formation. A stable mud weight window for Well-A was modeled and recommended based on this event. The stable mud weight window recommended was approximately 1.2 to 1.3 s.g, allowing for some minor shear failure. The avoidance of wellbore instability issues led to considerable reduction in time spent addressing drilling-related problems such as reaming, lost circulation, tight spots, etc., thus reducing nonproductive time (NPT) and rig cost.