Abstract
Abstract
Hydraulic fracturing and multistage fracturing have been intensively investigated, focusing on fracturing hardware and the operation itself and how to perform the fracturing operation efficiently and correctly. However, few studies have investigated wellbore temperature, pressure profiles, and the tubular stress status in a hydraulic-fracturing operation, especially stage-by-stage fracturing operations, where a sequence of multiple operations is performed above the end of the operation string (work-string, coiled tubing, etc.). This study investigates such a stage-by-stage hydraulic-fracturing chain scenario.
To model sequential stage-by-stage hydraulic fracturing, the temperature and pressure profiles of a prior operation are applied as the initial conditions for the next operation. For each operation, a plug is simulated at the fracturing depth in the operation string, and in the annulus two packers are simulated above and below the fracturing depth. This plug acts as a pressure and flow barrier, with the working fluid flowing above the plug, and the remaining fluids from prior operations non-flowing below the plug. There may be multiple non-flowing regions below the current active plug from previous operations. The heat transfer above and below the plug are different. Convection is dominant above the current active plug, while conduction and natural convection are dominant below it.
This study considers the differences around the plug and discovers that it acts as a pressure barrier and induces pressure discontinuity around it. The pressure difference around the plug induces axial stress discontinuity on the operation string over the plug. Similar behavior is also observed around the packers in the annulus, inducing stress discontinuity on the plug and casing. These behaviors may induce casing and tubing failure near the plug and packer. The temperature around each plug also undergoes a large change, but not as sharp as the discontinuity. The temperature below the current active plug, for each region formed by two plugs, finally converges to the geothermal temperature. That is to say, the temperature decreases or increases if the wellbore temperature is above or below the geothermal temperature when the corresponding fracturing operation is completed. This is important, especially for HT/HP well, because the temperature change could result in additional thermally induced fractures, as well as thermally induced stress on the casings and work-string, which impact the safety of the strings.
This study shows that operation type (fracturing, injection, circulation) and sequence could significantly affect the wellbore temperature and pressure profiles in a fracturing operation, as well as the inlet fracture parameters, and finally the fracturing quality. Simulating the wellbore temperature, pressure, and stress profiles during stage-by-stage hydraulic fracturing can certainly assist a more comprehensive wellbore structure design and operation process design.