Abstract

AGOCO is facing tremendous challenges to maintain the production of the mature giant Messla field. Water breakthrough, well shut-in due to sanding and reservoir depletion are among the problems causing a decline in the production. In order to maintain production, or even improve it in the near future, new technology and methodology are required to address the current challenges. Geomechanics, through a full characterization of mechanical properties and state of in-situ stresses of the reservoir and overburden formations, plays an essential role in achieving these objectives. This geomechanical knowledge forms a mechanical earth model (MEM) of the field that will enable appropriate technology to be deployed in the field.

To this end, a MEM was constructed by following an integrated process that includes data auditing, geomechanics laboratory testing, determination of mechanical properties and geostresses, and calibration and validation of the model.

The MEM was subsequently applied to improve drilling, production and reservoir management of the field. Sanding prediction was carried out to determine sanding risks in the field and based on the outcome of sanding prediction, sandface completions were optimized to not only minimise the sanding risk but also improve production. Wellbore stability analysis on underbalanced drilling and horizontal drilling was conducted to minimize potential risk to applying the technology to develop the depleted field. And under the guidance of the model, optimization of hydraulic fractures was carried out to help to achieve ultimate goal for maximizing the value of the asset and the investment in drilling and completion of the field.

Introduction

The giant Messla field is a Lower Cretaceous sandstone reservoir located in the southeast portion of Sirte Basin in Libya. It is about 500 km southeast from Benghazi, the second largest city in Libya, and approximately 40 km NNW of the Sarir Field.1 The Messla field includes DD area (the northern part of the field) and HH area (the southern part of the field).

The Messla field was discovered in 1971 and commercial production was started in the same year. As a mature field after production of 37 years, Messla exhibits many problems that are quite often seen in other mature fields - production decline, water breakthrough, well shut-in due to sanding, and reservoir depletion. In 2005, a geomechanics and sanding study was accomplished to evaluate potential sanding risk and optimize sandface completions in the DD area of the field. Following the study, pertinence of carrying out an integrated geomechanics study on the HH area was starting to emerge as AGOCO recognized that the new technology will be the key to maintain or even improve production of the field in long term and integrated geomechanics study will provide technical guidance on selecting, evaluating and implementing appropriate technology. To this end, in 2007 an integrated geomechanics study was conducted to establish a complete understanding and characterization of mechanical properties and state of in-situ stress of reservoir and overburden formations in Messla HH field. The findings of this study may be used for well design, completion and sand control (sand exclusion), production optimization, and reservoir management.

The study started with construction of MEM, which intended to capture geomechanics knowledge in the field. To ensure the reliability of the MEM, the input data were audited, the data gap was spotted and filled, the MEM parameters were calibrated, and the MEM was validated.

Based on the constructed MEM, sanding prediction was carried out to delineate sanding risk of the field. This provided a solid basis of sandface completions optimization, as previous experience indicated that attempts to run new completion designs without knowing the cause of the sanding and understanding the risks would be costly, and would be likely to fail.2,3

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