| dc.description.abstract | Since the inception and use of matrix acidizing to stimulate wells, the oil and gas industry has continued to develop new and improved fluids and materials for chemical diversion. However, the effects of chemical diversion are still inconsistent, difficult to diagnose, and difficult to predict. This dissertation seeks to develop a method for designing batch diversion treatments of the recently developed diverting agent, Polylactic Acid (PLA), even when the permeability profile of the well that is being stimulated is unknown. In the last decade, there have been several publications describing the use of PLA as a diverting agent in multistage matrix acidizing treatments. The aliphatic polyester is particularly useful as a diverting agent because it hydrolyzes in the presence of heat and water, leaving no residue in the formation, thus negating the need for any clean-up fluids. Most of the publications about PLA diversion focus on the physical and chemical attributes of the diverter and attempt to demonstrate its effectiveness via field trials, but few have investigated how PLA creates resistance to fluid flow. In this study, the mechanisms by which PLA can cause diversion during a multistage treatment are analyzed using laboratory experiments. The results from these experiments are used to develop a model that describes how local injectivity is affected based on the amount of diverter that is deposited.
This model is then incorporated into a near wellbore simulator and validated using published results. Finally, a geostatistical tool, sequential indicator simulation, is used to characterize the near wellbore permeability profile using permeability profiles from neighboring wells. This kriging-based interpolation technique is used because permeability profiles are seldom known with a high degree of accuracy, but it is more common to know the statistical distribution of the permeability based on neighboring wells. These experiments and simulations have revealed that PLA causes diversion very differently if it fills wormholes versus if it builds a filter cake at the surface. A greater resistance to flow can be achieved if the diverter bridges the wormhole opening and builds a filter cake on the sand face as opposed to filling the wormholes. It is also shown that even if PLA fills the wormhole, further acid injection causes the dominant wormholes to continue to propagate. The simulations have also shown that the use of PLA as a diverter can create a more uniform skin factor profile, but it can worsen the total skin factor. The method to design batch PLA diversion treatments, when the permeability distribution along the well is not known, was applied to a well in a synthetic reservoir that contains real well log data. The method was able to identify an optimal batch size of PLA based on the predicted cumulative well production. The results also demonstrate that the location of the high permeability streaks relative to the heel and toe have an impact on whether the application of diversion could improve the cumulative production of the well or not. Finally, the results show that if there is a mix of limestone and dolomite along the well, if the dolomite has a low permeability relative to the limestone, a diversion treatment tends to have a more significant impact on the cumulative production. | en |
