Evaluation of wellbore stability during drilling and production of open hole horizontal wells in a carbonate field

Drilling horizontal wells is a common practice for Saudi Aramco in most of its oil and gas reservoirs of Saudi Arabian clastic and carbonate fields. The field in hand with its two reservoirs is no exception in regards to these field development plans. While previously all wells in this field were cased and perforated, during the planning stage for increasing production, the question raised was whether openhole horizontal well completion is feasible over the life of the field (i.e., when taking near-wellbore drawdown and far field production-induced reservoir depletion into consideration. The direct benefit would be that openhole completion greatly reduces the development costs for the 300+ production wells planned for the field. A rock mechanics study was undertaken to provide a comprehensive understanding of wellbore stability of openhole horizontal wells throughout their life span from drilling through production during field development. Two objectives identified for the study were: 1) assessment of wellbore stability and critical drawdown rates during production to avoid well collapse, and 2) the optimal well deviation, azimuth and required mud weight during drilling to minimize wellbore instability problems. To increase accuracy of the results and greatly reduce uncertainty, cores from both reservoirs were retrieved in order to have representative samples of the formations of interest. A testing program was undertaken to determine the static and dynamic mechanical properties, compressive rock strength, rock failure characteristics and thick-wall-cylinder strength. The effect of water on rock strength was tested as well to evaluate if water encroachment poses additional risk to the mechanical integrity of the formation. In addition, the required geomechanical model - in particular in-situ stress field, magnitude and direction - was determined from several data sources: stress-induced wellbore failure analyses (from oriented caliper and, wellbore image log analyses, microfrac testing, direct pore pressure measurements, wireline log data and analysis of the general regional stress information for the area surrounding the field. The study showed that openhole completion is feasible for most well azimuths in both reservoirs. However it was determined that the tar bearing intervals of both reservoirs are not competent enough to be completed openhole due to the risk of wellbore collapse. The recommendation was therefore to avoid the tar bearing intervals and to consider casing those zones as applicable. The rock strength showed minimal effect as a result of exposure to water; therefore, water flooding will not be a concern from a wellbore integrity point of view. A field-specific compressive rock strength-wireline sonic log correlation was developed and calibrated with results from the lab tests. The flank wells tolerate more drawdown pressure than crest wells, due to higher rock strength in the flank as compared to the crest. Additionally, it is recommended that the wells be drilled in the direction of minimum principal horizontal stress (O_hmin), to maximize borehole stability and minimize required mud weights during drilling and completion. The results from this extensive study were incorporated into Saudi Aramco's reservoir management decision tree.