Fracture Plugging by Novel Thermally Activated Lost Circulation Materials

This paper examines the fracture plugging performance of novel thermally activated lost circulation materials (TALCMs) that can be used to effectively deal with severe lost circulation events. TALCMs are activated by a specific threshold temperature which can make the medium in which they are incorporated (such as cement slurries) unpumpable and thereby highly plugging in formation vugs and fractures. To clearly demonstrate this temperature-activated mechanism and fracture plugging behavior, larger scale rock mechanics tests were conducted on rock samples containing artificial fractures with apertures in the range of 1 to 3 mm. Pyrophyllite rock was chosen as the test medium to represent low-permeability rock formations encountered in a variety of wells (oil, gas, geothermal, carbon storage, etc., i.e. any well type that might suffer from severe losses in vugular/fracture systems). Lost circulation events in these types of rocks are generally more difficult to deal with, as they lend themselves poorly to wellbore strengthening techniques. Cylindrical rock samples with the desired fracture dimensions were prepared, placed in a temperature-controlled oven maintained above the TALCMs activation temperature to enable their intended function, and subsequently mounted in the triaxial equipment. Fluid formulations containing TALCMs were then circulated through the borehole at a temperature below their threshold temperature. The flow through the fracture was monitored to assess whether effective fracture sealing was occurring. Note that the TALCM fluid experience an increase in temperature to above their threshold temperature, with associated plugging consequences, when flowing through the fracture. Complete sealing of the 1 mm and 2 mm aperture fractures was observed in the tests, while the majority of the (short) 3 mm fractures also became sealed. Pressure testing of the 2 mm aperture fracture samples showed an ability to withstand significant differential borehole pressure. This work confirms that temperature-activation can be used as an effective new mechanism to cure severe losses in fractured or highly vugular media through the use of novel temperature-tunable TALCMs. Temperature management strategies are also presented in this paper to show how bottomhole circulating temperature (BHCT, kept below the TALCM threshold temperature) can be effectively managed in relation to bottomhole static temperature (BHST, above the TALCM transition temperature) to make the deployment of TALCMs most effective, with illustration using Utah FORGE geothermal well data.