Combination of Chemical Diverters and Degradable Fiber Technology Enhances the Success of Stimulation in Complex Carbonate Environments

Abstract

Abstract Due to damage caused by drilling and completion operations, most wells in the Jurassic carbonate reservoirs in North Kuwait require acid stimulation after perforating to either recover or enhance the well productivity. To date, production responses after stimulation treatments have been mixed, depending on the formation, the type of treatment and the volume and the chemistry of stimulation fluids. Additionally post-stimulation production logging surveys in many wells typically have shown poor stimulation efficiency with only a short perforation interval contributing to a majority of the production. Thus, it becomes crucial to develop general engineering guidelines and address treatment diversion issues for future stimulation work. This paper presents an engineering process for optimizing matrix stimulation and acid fracturing treatments according to laboratory tests and well performance evaluation in more than 20 wells in the deep Jurassic carbonate reservoirs in Kuwait. The paper also introduces a new stimulation technique that combines chemical diversion (viscoelastic surfactant based diverting acids) and degradable fiber technology to enhance leakoff control and diversion capability of the chemical solution into the complex naturally fractured carbonate formations. The technique improves leakoff control into large natural fractures, dominant wormholes and high permeability zones, and results in deeper acid penetration and more effective vertical coverage. The degradable fibers completely decompose with time and temperature after the treatment has been pumped. Field results have shown significant improvement of post-stimulation production after the general engineering guidelines were implemented. Application of this new stimulation technique has been evaluated through the production surveys and performance before and after the treatment. Initial results on the first well showed an eightfold increase in the productivity index. Background The North Kuwait Jurassic Fields include Sabriyah (SA), Raudhatain (RA), Northerwest Raudhatain (NWRA), Umm Niqqa (UN), Bahrah (BH) and Dhabi (DA) (see Fig. 1). Target reservoirs include the Jurassic carbonates of Middle Marrat (MM) and Najmah/Sargelu (NJ/SG) formations with TVD of 14,000–15,000 ft. Reservoir pressure and temperature are about 11,000 psi and 275°F respectively. The area is structurally very complex with NW-SE and NE-SW trending faults that tend to create different compartments. The depositional setting ranges from deep to shallow marine, including a supratidal environment (sabkha), and adds further complexity to the field development. The Jurassic carbonates (limestones and dolomites) and evaporites are highly heterogeneous both laterally and vertically, have a variety of pore types and are characterized by low porosity (range 0–22%, average 1.8–3%) and permeability (range 0–119md, average 1–2 md). Fractures appear to play a major role in production based on well performance studies. The MM formation is generally considered as pure with >90% of calcite and trace amount of silts and clays, whereas NJ/SG formation is generally considered as dirty with a high content of insoluble mineral, i.e. quartz, silts and clays, etc. Table 1 shows X-Ray Diffraction analysis on core samples from six wells. Reservoir fluids are critical in nature (both gas condensate and volatile oil) and are not distributed uniformly across the project area.