Engineered Wellbore Strengthening Solution Enables Drilling of Depleted Reservoirs by Expanding the Drilling Window
Authors: Saba Thony , Adilin Syahir , Dey Rahul Sukanta , Kadir Aisar 
Published: February 2020

Wellbore Instability and Lost Circulation continue to be the costliest problem encountered during wellbore construction. While conventional remediative treatments can decrease drilling fluid losses, increasingly challenging enviornments like mature and depleted fields require a wellbore strengthening approach to prevent these occurences and enable effective well construction. An engineered wellbore strengthening package was used to drill depleted sandstone reservoirs and enabling the client to drill the well, which was classically thought to require a Managed Drilling Pressure (MPD) solution. The paper will outline the calculated approach taken to prove the concept and the overall performance in the well.

In the case of mature, depleted reservoir, as the reservoir pressure declines so does the Fracture Gradient (FG) and this becomes an issue where the reservoir is overlain or interbedded with relatively impermeable shale layers. Drilling Fluid Densities sufficiently high to stabilize the shales can generate overbalances in accompanying depleted sands that are so high that the wellbore pressure exceeds the FG. In such scenarios high overbalances increase the likelihood and severity of lost circulation.

A geo-mechanical model was developed for the depleted sandstone section to calculate the pressure-induced fracture geometry. The rock behavior was established using the stress directions and its physical characteristics such as Young's modulus and Poisson's ratio. Using the software simulation, an appropriate Wellbore Strengthening solution package was designed specifically for this section. The Wellbore Strengthening Material (WSM) volumes and mud system were decided to be the best-fit for the application to provide the best chance for success in the well. A strategy was developed to replenish the WSM in the active fluid system by evaluating the solid control systems and knowledge of the attrition rate of specific materials in the Wellbore Strengthening blend. The treatment of these materials was applied to reinforce and protect the target zone against higher drilling fluid densities and Equivalent Circulation Densities (ECD's) during drilling.

The Wellbore Strengthening Materials (WSM) blend design provided excellent performance on expanding the drilling window for the depleted sandstone reservoirs. The window was extended up to 2 ppg from the initial calculations. The troublesome section was completed ahead of schedule and with reduction in total Authorizations for Expenditure (AFE) for the client. The overall downhole fluid losses were reduced by 75% which presented additional efficiency for the client. A well-engineered approach for WSM ruled out the use of MPD solutions which can lead to installation of additional equipment on the rig site. The WSM package included easy to mix particulate materials on the rig site. Their depletion in the drilling fluid for the calculated loss over the solid control equipment proved to be the main driver for the success seen in this well.