Abstract

This paper highlights the development of a coupled poroelastic geomechanical and fluid flow model which incorporates field and lab data with the objective to constrain the full in-situ stress tensor and rock strength in order to predict the stability of open hole horizontal completions during reservoir depletion.

Results of a four-year comprehensive testing and monitoring program conducted to assess the extent of hole instability during shut-in and flowing periods1 indicated that there was no immediate hole collapse. However, the study revealed the need to assess the long-term impact of reservoir depletion and pressure drawdown on wellbore stability.

The results of this study indicate that the in situ stress state can be characterized by a normal faulting environment with low differential stresses in which the maximum principal stress is approximately equal to the overburden. Furthermore, a detailed analysis of wellbore stability during production supports openhole completion for horizontal wells under the condition that reservoir depletion is limited to a maximum pressure drop of 1,500 psi. This finding is independent of well azimuth. Pressure drops exceeding 1,500 psi in the reservoir are likely to cause considerable wellbore instabilities. These results were achieved under the assumption of moderate to more pronounced amounts of drawdown (500–1000 psi) in the near wellbore region. The study also highlighted that laboratory-derived rock strength values from triaxial tests, are low and are not consistent with the drilling and production experiences to date in the field. Rather, the formation appears to behave in a plastic manner that strengthens the wellbore.

Introduction

Hole stability concerns in the Shu'aiba reservoir, Shaybah Field, Saudi Arabia, first surfaced during the drilling and logging of two development vertical evaluation/production wells where a logging tool was stuck in one well due to tight hole, and indications of tight hole were encountered while drilling another well. The two incidents signaled the need to investigate hole stability in the Shu'aiba reservoir. A review of all the vertical delineation wells drilled in the 1960's for problems associated with hole fill and/or collapse during drilling and production found no conclusive evidence for stability problems.

Cores taken in the mud-rich, high porosity rock of the Shu'aiba formation have been described as having a "toothpaste-like" texture and behavior. Preliminary laboratory rock mechanics studies indicated that the Shu'aiba carbonates are mechanically weak with the majority of the rocks tested yielding very low strength values (less than 2000 psi) when compared to samples from other carbonate reservoirs.

In light of the gathered field and geomechanics data, a comprehensive hole stability monitoring program was formulated and initiated with the objective to investigate the extent and implications of hole stability on field development and deliverability. Results of which are summarized in SPE 565081 indicating, at least in the short term, no impact of production on wellbore stability.

Furthermore, a long term study, which is the focus of this paper, was initiated to constrain the full in situ stress tensor (i.e., orientation and magnitude), reservoir pore pressure, and rock strength in order to build a geomechanical model of the Shaybah Field and predict the stability of openhole horizontal wells during reservoir depletion. To achieve these goals, a broad suite of available data from wells drilled in Shaybah Field were utilized such as electrical FMI image data, four-arm caliper logs, minifracs, wireline logs (e.g., density, neutron, and sonic logs), and pore pressure information obtained by direct measurement were studied. This data - sufficient to constrain the full stress tensor and pore pressure - was then augmented by information on uniaxial compressive strength derived from laboratory measurements and log data to build the full geomechanical model, which was then used as a basis to predict wellbore stability during reservoir depletion.

This content is only available via PDF.
You can access this article if you purchase or spend a download.