Geomechanics Assessment For Mature Fields - A Multidisciplinary Approach

Abstract

The redevelopment of mature oil fields has been, and will increasingly be, an important subject. Oil recovery from mature fields can be boosted by drilling infill horizontal wells and through the use of enhanced recovery and production optimization techniques. Years of production and significant reservoir pressure declines in mature fields could cause considerable changes in the in situ horizontal stresses in the reservoir sections and surrounding rocks. Reduction of in-situ stresses could result in narrow drilling margins, increased risk of sand production and fault reactivation which in turn could make the drilling and completion of infill wells in depleted reservoirs operationally challenging. Therefore, a robust knowledge of rock mechanical properties, formation pressures, the magnitude of in-situ stresses and their evolution with production and injection are essential for successful drilling and completion of new infill wells in particular and the redevelopment of mature fields as a whole.

In this study, case histories from two mature oil and gas fields in Asia Pacific region will be presented to highlight how a systematic geomechanical evaluation can be used to address the drilling and completion challenges such as narrow drilling margin of infill extended-reach wells, mud losses through conductive natural fractures and critically stressed faults intersected by high-angle infill wells. A present-day geomechanical model was developed and calibrated with more than thirty years of drilling, logging and production data including laboratory rock mechanics experiments and recently conducted water injection tests. The model was used to design the new high-angle infill wells trajectories, casing setting depths and mud weight requirements. The model was also used to assess the critically stressed faults within various compartments of the block as well as to address the well completion challenges associated with sand production risks.

The results showed drilling high-angle wells in SHmax direction would be more challenging because of high risk of encountering mud losses during drilling and cementing with elevated bottomhole pressures exceeding the fracture initiation pressure (FIP). Intermediate and production hole sections of infill wells would have narrow safe drilling margins of less than 0.8 pounds per gallon (ppg) due to a high depletion levels of deeper sandstone reservoirs and encountering weak shale intervals in the same hole section which require higher mud weights to withstand the possible shear failure. The subsequent drilling campaign of planned infill oil producers and water injectors in the field has been successful due to good drilling practices, real-time monitoring and update, applying the mud programs consistent with wellbore stability assessments and careful bottomhole pressure control.

Furthermore, a poroelastic analytical sand production prediction approach was used to investigate the risk of sanding in infill wells under high depletion conditions and increased sanding risks from cased hole perforations associated with water injection. Historical production data were used to calibrate the analytical sanding model. Advanced Thick-Wall Cylinder (ATWC) tests were conducted on sandstone core samples saturated with mineral oil and brine to characterize the amount of rock weakening in the presence of formation water, which showed target sandstones may weaken by 13 - 16% when they are exposed to brine compared to the mineral oil. The sanding risk assessments showed a very low risk for the target oil reservoirs over the field life. However, sanding risk was found to be slightly higher with further depletion from the current reservoir pressures, particularly with the rock weakening effects under water production and injection conditions. This results are used to manage the flowing bottomhole pressure during production at different depletion levels to stay within the sand-free envelope. Production data from the field shows the results of this study are still valid with no sanding observations within the sand-free envelope.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018