LCM Plugging Effect on Producing Formations During Drilling Naturally Fractured Sandstone Reservoirs

Abstract

Abstract Naturally fractured sandstone reservoirs are susceptible to drilling mud damage both at the fracture and matrix level. This problem becomes especially severe as reservoir pressure depletes due to: (i) the loss of backpressure preventing static and dynamic mud losses and; (ii) loss of energy for well clean up. Although several completion practices are covered by the experimental techniques described, of special interest in this study is cemented and perforated liners which can have some particular constraints if not properly designed and executed when drilling and completing mature naturally fractured sandstone reservoirs. While drilling, the overbalance pressure exerted by the drilling mud on producing formations is a means for avoiding wellbore stability problems. When the reservoir pressure is close to its initial value, the near-wellbore region affected by mud losses and invasion is normally restricted to a few inches. Additionally, the reservoir often has enough energy to overcome filtrate damage within the rock matrix and inside natural fractures. This condition changes as reservoir becomes depleted, as a higher drilling overbalance occurs for the same mud weight, and reservoir energy for subsequent well clean-up is reduced. The following study presents an experimental evaluation of synthetic oil-based mud (SOBM) drilling damage on naturally fractured reservoir cores. Two conditions were simulated: first, a condition in which mud invasion takes place into open natural fractures, followed by simulated under-balanced perforating and post draw-down stimulation. A second scenario is then considered in which simulates mud invasion into open fractures, but which is then followed by on-balance perforating and stimulation, and only after this is a draw-down imposed on the sample. Results from the study indicate that very high levels of formation damage are caused when natural fractures get plugged and/or closed-over by mud filter-cake and lost circulation material (LNM) solids. Fluid pressure drawdown in the near-wellbore region and induced fracture closure pressures can, in the field, essentially create a compacted "new rock", with very low permeability and very high tensile stress. In this particular case study, this last condition is evident in the very low injectivity of the stimulation fluid and the very low value of regained permeability. When natural fractures are stimulated before imposing a drawdown, very good return permeability is achieved with an even higher value after closing fractures and re-stimulating.