Application of Real-Time Geomechanical Modeling to Prevent Wellbore Instability While Drilling Horizontal Wells with Extended Reservoir Contact
Authors: Gomez Ernesto Gomez Samuel , Rivas Raider , Ombe Ebikebena , Ahmed Sajjad 
Published: December 2021

Background

Drilling deviated and horizontal high-pressure, high-temperature (HPHT) wells is associated with unique drilling challenges, especially when formation heterogeneity, variation in formation thickness as well as formation structural complexities are encountered while drilling. One of the major challenges encountered is the difficulty of landing horizontal lateral within the thin reservoir layers. Geomechanical modeling has proven to be a vital tool in optimizing casing setting depths and significantly increasing the possible lateral length within hydrocarbon bearing reservoirs. This approach ultimately enhanced the production output of the wells.

In a particular field, the horizontal wells are constructed by first drilling 8 3/8" hole section to land about 5 to 10’ into the impervious cap rock just above the target reservoir. The 7" casing is then run and cemented in place, after which the horizontal hole section, usually a 5 7/8" lateral, is drilled by geosteered within the target reservoir to access its best porosity and permeability. Due to the uncertainty of the cap rock thickness, setting the 7" liner at this depth was necessary to avoid drilling too deep into the cap rock and penetrating the target reservoir. This approach has its disadvantages, especially while drilling the 5-7/8" lateral. Numerous drilling challenges were encountered while drilling the horizontal lateral across the hard cap rock. like severe wellbore instability, low ROP and severe drillstring vibration.

To mitigate the challenges mentioned above, geomechanical modelling was introduced into the well planning process to optimize the 8 3/8" hole landing depth within the cap rock, thereby reducing the hard caprock interval to be drilled in the next section. Firstly, actual formation properties and in-situ rock stress data were obtained from logs taken in previously drilled wells in the field. This information was then fed into in the geomechanical models to produce near accurate rock properties and stresses values. Data from the formation fracture strength database was also used to calibrate the resulting horizontal stresses and formation breakdown pressure. In addition to this, the formation pore pressure variability was established with the measured formation pressure data. The porosity development information was also used to determine the best landing depths to isolate and case-off the nonreservoir formations. Combined with in-depth well placement studies to determine the optimal well trajectory and wellbore landing strategy, geomechanical modelling enabled the deepening of the 8 3/8" landing depth without penetrating the hydrocarbon reservoir. The geomechanical models were also updated with actual well data in real time and allowed for the optimization of mud weight on the fly. This strategy minimized near wellbore damage across the reservoir section and ultimately improved the wells productivity index.

Deepening of the 8 3/8" landing depth and minimizing the footage drilled across the hard and unstable caprock positively impacted the overall well delivery process from well planning and drilling operations up to production. The following achievements were realized in recent wells where geomechanical modelling was applied: The initiative helped in drilling more stable, in-gauge holes across the reservoir sections, which were less prone to wellbore stability problems during drilling and logging operations.Downhole drilling tools were less exposed to harsh drilling conditions and delivered higher performance along with longer drilling runs.Better hole quality facilitated the running of multistage fracture completions which, in turn, contributed to increase the overall gas production, fulfilling the objectives of the reservoir development team.The net-to-gross ratio of the pay zone was increased, thereby improving the efficiency of the multistage fracture stages, and allowing the reservoir to be produced more efficiently.