Particulate Wellbore Fluid Strengthening Methodology. Design and Application in an Offshore Vietnam Severely Depleted Sand Reservoir

Abstract

In the mature Cuu Long basin, offshore Vietnam, the dynamic reservoir model projected the pore pressure's depletion as high as 4,723psi countering virgin pressure (up to 8,500psi) for one particular "gas-condensate" well. Coupling with expected deviation of 38° inclination and un-disturbed temperature of up to 170°C, posed a considerable challenge for this well. High bottom hole pressures can potentially exceed the formation fracture gradient resulting in tensile failures and well fractures. Wellbore strengthening (WBS) techniques are used to allowing depleted reservoir formations to be drilled with lower risk. Hoop stressing is a method to increase wellbore stress so that a controlled tensile failure occurs at an elevated wellbore pressure; thereby, strengthening and expanding the wellbore stability window. This method uses material introduced into the mud system to seal off induced fractures and in effect, increase and maintain the hoop stress in the wellbore when drilling. The inherent challenges include understanding the effect of depletion to the change of induced fracture pressure and fracture width (fracture aperture) also Rock elastic properties (Young's Modulus, Poisson Ratio), well design (well architecture, hydraulic), testing, and validating wellbore strengthening by the material. A proprietary software was used to model a probabilistic distribution of the potential induced fractures given the in-situ conditions. Modelling involved using location-specific geo-mechanical information, planned drilling conditions, and parametric sensitivity to predict induced-fracture widths. Fracture widths from 500 to 750μ were determined based on modelling and used as the baseline for testing validation. Testing was performed using a Fully Automated Advanced Slot Tester (FAAST) to optimize WBS packages to seal off the predicted fracture widths with testing pressures reaching 4,000psi. A range of slot sizes were tested to evaluate and validate performance of various particulate WBS packages, with focus on 500 and 750μ sizes. The final optimized package included both proven sized strengthening material as well as finer bridging material. Field execution of the WBS techniques consisted of introducing and maintenance of the optimized package to the drilling fluid system on a continuous basis while drilling different hole sizes (12-1/4″, 8-1/2″, or 6″ as contingent section) through the depleted reservoirs through the usage of concentrated engineered pre-mixed pills in reserve to optimize logistics. Replenishment of active concentrations was carefully monitored to counter the removal of the material by the surface solids control equipment as well as the material consumption in the induced fractures. Drilling the reservoir section with the wellbore strengthening and bridging technique was successful. Implementing this strategy delivered a stable wellbore while drilling and the liner or casing run to bottom successfully. Field measured pore pressures performed, determined a maximum differential overbalance pressure of 3,547psi recorded for the well with equivalent circulating pressures exceeded the fracture gradient by as much as 790psi. These field validated results support and demonstrate that the effective use of wellbore strengthening methodology can create an operating window thereby reducing operational risk in severely depleted formations.