Wettability alteration of carbonate rocks to alleviate condensate blockage around gas-condensate wells
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Over the recent decades, gas-condensate reservoirs have attracted increasing attention from many energy suppliers around the globe. The production and improved recovery techniques to extract more hydrocarbons from these reservoirs are still a demanding and challenging subject. Two unique and complex effects that prevail in gas/condensate flow behaviour near the wellbore are: i) negative inertia, decrease in relative permeability with increasing velocity and ii) positive coupling, increase in relative permeability with increasing velocity/decreasing IFT. When pressure near the wellbore falls below the dew-point the wetting condensate that drops out can block the gas flow paths. The water blockage can also cause severe productivity losses. In the last decade, wettability alteration of the reservoir rocks from liquid- to intermediate-gas wet conditions, using appropriate liquid-repellent chemicals, has been proposed as a new solution to mitigate the liquid hold-up around the wellbore on a relatively permanent basis compared to other common remedial techniques, e.g. solvent injection and hydraulic fracturing stimulation. Despite relatively considerable number of investigations in the literature dedicated to application of such chemical treatments in gas-condensate systems, there are still serious and important questions about the reliability of the results. That is, in all these investigations, conventional gas/oil fluids and/or procedures and conditions not representative of gas condensate reservoirs have been employed. The research work presented in this thesis is devoted to providing a better understanding of the application of such chemical treatments in gas-condensate systems and address the crucial limitations and benefits associated with this stimulation technique. The investigations performed are also aimed at finding appropriate wettability modifiers for carbonate rocks, which compared to sandstones, have received less attention. The current research study can be divided into three main parts. In the first part, results of initial screening tests performed to identify an optimized treatment solution for carbonate minerals are discussed. The conventional fluid systems such as decane-nitrogen are employed for this purpose. Initially, the static contact angle data are used to examine the performance of fifteen fluorinated chemicals, out of which four liquid-repellent agents have been short listed. The brine compatibility, spontaneous imbibition and unsteady-state displacements tests are then employed to investigate the impact of a number of important parameters including: chemical concentration, chemical particle size, solvent type, temperature, rock permeability and brine salinity. The final treatment solution developed in this part, i.e an anionic fluorosurfactant carried with methanol, is further evaluated to understand its impact on improving the fluids mobility as well as its durability. In the second part of this thesis, the performance of the optimized wettability modifier developed in the first part, is evaluated in presence of gas-condensate fluids. For this purpose, initially high pressure (and in few cases high temperature) contact angle measurements using synthetic binary- and multi-component gas-condensate mixtures are conducted. These tests, for the first time, reveal that the wetting characteristic of the treated carbonate surface is significantly dependent on the interfacial tension and molecular composition of the system. That is, by increasing the interfacial tension and/or number of carbon atoms in the system the contact angle increases. The generality of these new findings are further confirmed by performing a number of unsteady-state displacement tests. The ultimate effect of the wettability alteration is finally quantified by measuring series of steady-state relative permeability data points under various IFT and velocity conditions. The new findings demonstrate that chemical treatment performance is significantly affected by the thermodynamic and flow conditions of the gas-condensate system. In the final part of this work, a simulation exercise is carried out to complement the experimental results and provide better practical understanding of application of this chemical treatment technique. Here, a number of single-parameter simulations are initially performed, which in particular look into the favourable performance of chemical treatment versus solvent injection, water- versus alcohol-based solution and chemical treatment in mature fields with large condensate banks. In the second section of this part, the simultaneous impacts of five important parameters on well productivity are scrutinized employing statistical approaches. The five selected parameters are treatment radius, treatment uniformity, improvement factor of relative permeability, permeability damage and treatment durability. Accordingly, a Matlab-based computer programme is developed to facilitate the simulation of 1080 multi-parameter simulations using a full-factorial experimental design. The relationship between the parameters and their impact on treatment performance are then analysed using the fitted linear response surface models. It is demonstrated that the kr improvement factor has the largest positive impact on CT performance. On the other hand, permeability damage (PD) is the only negative parameter with a significant adverse impact on the CT performance. In summary, the new findings from the study provide valuable insights into the practical application of chemical treatment and offer useful guidelines on designing more effective wettability modifiers.