During the process of secondary sand fracturing, the proppant volume is divided into two parts. At first, pump the first part of proppant into the formation, stop the pump until the proppant settle and fracture close, then pump the second part of proppant. Secondary sand fracturing can form short and wide fracture with high conductivity, and it has good effect on restraining the excessive extension of fracture height. The technology has been applied successfully in China, and the stimulation effects were great. But the relevant theory on secondary sand fracturing is deficient, which makes the field fracturing design have great blindness.
Based on considering the changing of sand bank height along with the changing of time, this paper establishes the mathematic fracture extension model for secondary sand fracturing; considering the changing of fracturing fluid filtration, flowrate, proppant concentration and fracture modality with time, the proppant migration of secondary sand fracturing model is developed. By solving the simultaneous equations of fracture extension model, proppant migration model and fracturing fluid filtration model, the dynamic and static fracture modalities of secondary sand fracturing can be determined. Through program calculation, the result show that compares with conventional fracturing, the fracture length and height of secondary sand fracture decrease, but the width and conductivity increase. According to real reservoir characteristics, the optimized secondary sand fracturing technology has great stimulation effects in fields, so it is worthy of application and dissemination.
Although conventional fracturing technology plays an vital role in reservoir stimulation, it often causes a series of problems such as sand plug, water layer communication etc, which would lead to failure or poor effect of fracturing. As a new technology[1,2], secondary sand fracturing formed during practice of gas field fracturing in China. The secondary sand fracturing is different from refracturing[3,4], the proppant volume is divided into two parts and pumped into formations separately during the process of fracturing. After secondary sand fracturing, the fracture length obtained will be shorter and fracture width and the sanding concentration in fracture will increase relatively. Secondary fracturing is fit for the formations which need to control fracture height and improve fracture conductivity, it could also reduce the risk of sand plug. The relevant theories on secondary sand fracturing are still deficient now, which make the field fracturing design have great blindness.
According to the conventional fracturing design model[5,6] and the features of secondary fracturing, this paper studied the fracture propagation model and proppant migration model of secondary sand fracturing and analysed the effect of secondary fracturing in different combinations of formation parameters and construction parameters. Finally we analysed the successful application of secondary fracturing at a certain field.
Producing formation, cap rock and substratum are all ideal homogeneous elastic solid, but their mechanical parameters, such as Poisson ratio, elastic modulus and fracture toughness, are different;
The minimum horizontal stress in producing formation is less than the one in cap rock and the one in substratum.
