A Study On Physical Property Evolution Of Reservior Formation During Process Of Water Injection And Hydrofracture

Water injection and hydrofracture are two frequently used operations in the exploitation of oil and shalegas. Large quantity of water is injected into the formation during water injection operation to maintain reservoirs pressure and sweep oil or gas towards production wells. The rock of uncoslidated formation can be eroded to form wormhole which may increase the risk of wellbore damage. While the hydraculic fracture operation can create a channel for oil and gas to flow into wellbore, this process will greatly change the in-stu stress of formation and induce the reopening of micro natural fractures. The study on physical property evolution of the reservior formation in these two processes is significant for opration design.During long-term of water injection, wormhole can form due to the erosion of formation rock. The mechanism of wormhole initation and propagation is an importan problem. In the prsent study, we built a multi-physicis model, in which hydrodynamics, geomechanics and erosion were all considered. With a stochastic porosity reservoir model, the effects of initial weak plane on wormhole was studied. It was founded that the initial porosity is a key factor for geometry and propagation direction of wormhole. The parameters such as injection flux, erosion coefficient and injection pressure were investigated. The interactions of multi-perforations were studied throuth the change of their length and angle. It was founded that the wormhole is more likely to initiation at the cusp of longer perforations. With the decrease of angle, wormhole at the tip of perforations is restrained while that the wormhole area around wellbore increase.The well instability induced by well washing may increase the risk of wellbore damage. An erosion model was proposed through considering the coupling effect of plastic failure and erosion. Typical engineering cases were simulated for this process. Results show that the rock around wellbore and the cusp of perforation can be damaged in the process of well washing. This damage region promotes the erosion and the erosion weakens the strength of the new area with no damage. With the decrease of well washing pressure, the damage area increases. According to the results, through the control of water quality, increasing the strength of cement sheath and improving washing technology, the security of wellbore can be improved.The operation parameters of hydraulic fracture such as pumping rate, fracturing sequence and fracture spacing, etc can greatly affect the distribution of stimulated reservoir volume (SRV) or enhanced permeability area (EPA). The ultimate purpose of hydraulic fracture is to achieve economic and efficient gas production. In this paper, we built a bridge to link the reservoir stimulation process and production performance. Firstly, the EPA was calculated in the simulation of hydraulic fracture propagation. Then the EPA data was used directly to model the reservoirs for the prediction of gas production. Using this model, we studied the effect of some key parameters such as cohesion of formation, spacing, pumping rate and fracture sequence. Results show that EPA can connect around the cusp of fractures. Connected EPA acts as a high-speed channel and is beneficial to well performance. High pumping rate treatment greatly changes the stress field around hydraulic fractures and promotes the form of EPA. When fracturing with high pumping rate, the spacing can be properly increased. Simultaneous fracture treatment can greatly improve the conductivity of formation compared with sequential treatment. But for large spacing case, simultaneous fracture treatment has no advantage.In this paper, the physical property evolution of reservior formation during the process of water injection and hydrofracture was studied. The presented methods and obtained results may provide a theoretical base and technical support for operation design in water injecton and hydraulic fracture.