Abstract
Geomechanic can play a major role in designing successful waterflood operations. The objective of this paper is to discuss the Geomechanic workflow that aims to provide optimal injection pressures which maximize the injected volumes while avoiding the growth and propagation of hydraulic fractures in the reservoir. The success of waterflood development depends on a proper pressure injection design which requires knowledge of formation fracture pressures and rock properties. This paper shows how Geomechanic can be used advantageously to set appropriate injection constraints that can vary from one field to another and even within a field by itself. Within PDO it is now a common development practice to do a geomechanics studies to investigate the maximum allowable injection pressure such that no fractures will develop and grow (matrix injection conditions).
A semi-analytical wellbore equation is used (based on an elasto-plastic model) to predict a range of bottom hole pressures that can be used to avoid fracturing. Further, a Monte Carlo analysis is applied to get probabilistic results. Data from well tests and operational data like Hall plots were analyzed and used in this geomechanics workflow. Also, geomechanical modeling was carried out to estimate the rock properties (from logs) and the orientation and magnitude of the stresses. It is critical to understand the in-situ stresses in the formation, in particular the minimum horizontal stress. Therefore, the derived (estimated) data was complemented by measured data: pore pressure and the minimum horizontal stress (mini-frac data). Results were verified via a rigorous analysis of a number of waterflood campaigns. These confirmed that in general fracturing conditions can be (substantially) higher than the generally assumed minimum stress (gradient) or the minimum stress (gradient) in the formation that is to be injected.
As an example this geomechanics workflow that determines the maximum injection pressure will be applied to a reservoir (in Oman) that consists of carbonate and shale formations. The maximum injection pressures show variations across the formation. Based on the results of the analyses in this study limits for waterflood injection pressure were recommended for the future development and will have a direct impact on the economics of the planned field development in terms of recovery rate.
