Improve drilling efficiency and reduce costs

With an aggressive drilling campaign, Apache faced the tough task of improving drilling efficiency and practices to extend the life of the mature Forties field in a challenging North Sea environment. As field production declined, economic viability demanded a performance change. Appropriate remedial actions needed to be based on what was occurring downhole to avoid incorrect diagnoses and solutions that could lead to further damage.

Apache's drilling program in the UK sector of the North Sea involved infill targets from donor wells sidetracked near the base of the 9 5/8-in [245-mm] casing or step-out targets with sidetracks higher up existing wells. As the field matured, the safe mud window was becoming progressively narrower, making drilling more difficult.

In addition, the likelihood of losses increases in maturing fields as reservoir pressures decline. To reach targets farther from the platform, higher mud weights are needed to prevent collapse of the overburden shales, and hole cleaning becomes more difficult. Mud weight selection becomes a compromise of threading the needle between hole collapse and losses. Of the 94 boreholes drilled by Apache over a 5-year period, 45% were lost as a result of drilling or completion problems, and a BHA was lost in 40% of these failures.

Provide multiwall sonic workflow using new technology

Apache selected the Schlumberger petrotechnical services for geomechanics group, to lead a collaborative team to identify, predict, and prevent costly wellbore failures through an innovative combination of operations monitoring, geomechanics behavior analysis, and risk management, including the integration of seismic, well logs, and core measurements.

Schlumberger provided a multiwell sonic workflow using new technology that integrated a virtual seismic profile, the Sonic Scanner acoustic scanning platform, and core tests to describe anisotropy parameters in the overburden. The workflow allowed for marked improvement in fieldwide seismic calibration and geomechanical models.

To quantify anisotropic behavior, the team used the Sonic Scanner platform, borehole-seismic and core-log integration, and TerraTek rock mechanics and core analysis services for fieldwide calibration of sonic and seismic data.

Identified, predicted, and prevented costly wellbore failures

The workflow enabled the capture and classification of historic drilling events—key processes in determining causes of wellbore failure— to be performed systematically and efficiently in the fieldwide multiwell environment. This method helped to diagnose more than 120 wellbores for instability causes. The anisotropy results successfully refined wellbore stability predictions and seismic interpretations, prompting Apache to conduct similar anisotropy investigations for other overburden shales.

As a result of the knowledge gained during this collaborative project, NPT costs for one year were reduced by more than 60%, and further, no BHAs were lost after implementation of the team solutions. The closer collaboration between disciplinary experts and the field-proven practices was rapidly incorporated into future well planning and drilling operations, and will continue to help mitigate risks and reduce costs.