The nature of some water injection processes can cause unavoidable formation damage. In most cases, by optimal design of the process, the formation damage can be minimized or avoided. The estimation and assessment of formation damage on reservoir core samples is an important measurement which will be presented in this paper by monitoring rock permeability changes.
Laboratory tests were set up to investigate the effects of injection pressure, flow rate and also induced fracture on the rock sample with respect to formation damage. Reservoir core nature, structure and its heterogeneity make the phenomenon of formation damage more complex. In this research, selected rock samples from one of Iranian giant oil fields, which is undergoing water injection, were used as the laboratory physical model. Using injection water samples, different scenarios were performed on this model. Permeability reduction was observed for un-fractured samples: however, it was more interesting that no injection pressure build up was detected in fractured samples. It appears that fractures in sandstones, especially with weak cementation, can play the role of an agent for particle movement in causing damage to different parts of the formation. Over a long period of time, this phenomenon can cause more complicated situations in the reservoir, the degree of which being a function of several parameters, such as rock type and structure, pore size distribution and reservoir heterogeneity.
Introduction
Formation damage can cause a reduction of oil production in producing wells, and also decrease injectivity in injection wells. Consequently, changes in permeability, production pressure and injection pressure are used to evaluate formation damage. It is usually measured and analyzed in the field based on well performance changes using pressure transient analysis, during which permeability index variation in wellbore area is evaluated. It can also be evaluated in the laboratory by conducting special core flood tests and monitoring permeability changes at reservoir conditions.
It is usually difficult to precisely analyze and evaluate the origin, occurrence and extent of formation damage. Careful test design and monitoring is required, by experienced scientists and researchers, which is based on accurate definition, monitoring of induced formation damage to identify the various controllable /uncontrollable factors and avoidable/unavoidable factors that cause formation performance which is difficult to measure accurately.