Abstract
Drilling depleted reservoirs is a challenging operation due to significant mud losses and potential risk of wellbore collapse against shale or high-pressure formations if mud weight has been decreased to mitigate mud losses across depleted reservoirs. Wellbore strengthening is an efficient and reliable technique to enable a continuous drilling operation for formations with different pore pressures such as drilling depleted reservoirs with the high mud weight required to support shale or high-pressure reservoirs.
If wellbore strengthening has been designed correctly and successfully executed, it could eliminate the total number of casing stages required to drill formations with different pore pressures separately to avoid mud losses or wellbore collapse during drilling. Decreasing casing stages will ensure reaching the target reservoir with larger casing size, thus improving well productivity.
Wellbore strengthening technique includes the initiation of short fractures where bridging particles are used to seal and hold the fractures open and prevent any further propagation inside formation. The hoop stress around wellbore will be increased accordingly and enable drilling such depleted reservoirs with higher mud weights. This paper discusses the methodology and assumptions considered in wellbore strengthening design including fracture propagation and the optimum geometry required to yield maximum wellbore strengthening during drilling. The paper will also propose an optimized workflow to be used as guideline for this application.
Wellbore strengthening design includes two main parts: fracture width opening and particle size calculations. There are many different analytical methods which can be used to calculate fracture width including PKN, GDK, etc. This paper presents a case study where all methods were screened and compared to the results of a pseudo-3D fracture propagation model. The most appropriate and reliable analytical method to calculate fracture width distribution inside formation during wellbore strengthening has been identified for complex geomechanical environment such as stress anisotropy field.
Fracture half-length is usually assumed in wellbore strengthening design to allow the calculations of fracture width. This assumption is not always right, it is associated with high degree of uncertainties and may not represent the optimum fracture geometry. This paper discusses a new design approach where fracture geometry can be optimized including the optimum ratio between invaded and non-invaded zones and the optimum fracture geometry required to maximize wellbore strengthening during drilling depleted reservoirs. A robust, reliable, and optimized workflow is also suggested in this paper to ensure that all uncertainties associated with wellbore strengthening design have been considered and addressed.
