A Practical Method to Monitor Wellbore Strengthening Particle Concentration

Abstract The StressCage methodology of wellbore strengthening uses sized particles to bridge and seal induced fractures, increasing fracture resistance in permeable formations. The size distribution of the particles, or fracture prevention material (FPM), that are added to the mud is engineered to ensure correctly sized particles enter and bridge the induced fracture before it grows beyond its designed width and length. A minimum concentration of FPM is determined through a physics-based numerical calculation to derive a mud formulation based on the particle size distribution (PSD) and density of the selected mud additives. This minimum concentration and particle size distribution of FPM must be maintained at all times that the wellbore pressure exceeds the fracture gradient or previously generated fracture resistance of the formation in order to prevent the failure of the StressCage and the subsequent loss of wellbore integrity. A number of methods have been used to monitor FPM concentration in the mud. Most methods sample the mud returning from the well before arriving at the mud shakers, pass the collected mud sample through stacked sieves, remove the liquid mud coating the particulates using either centrifuges or drying ovens, and weigh the resulting residue to determine the return concentration. These methods can be inadequate and difficult to implement due to the requirement of specialized equipment on the rig (ovens, laser PSD, centrifuges, etc.), the need for additional personnel to conduct the mud monitoring activities on the rig, and the significant delay in obtaining the results. The lag time for results in deepwater wells can be 5 to 10 hours, depending on the time required to circulate the mud up the well and dry the samples. This delay can prevent operators from detecting when FPM concentration decreases to below the minimum required for drilling depleted sands. A simple and fast method has been developed that requires minimal special equipment. A reference sample is collected from a reserve mud pit carefully prepared with the target minimum FPM concentration. The mud is then run though a set of selected stacked sieves. The reference tare weights of the wet FPM collected in each sieve are used as a reference for monitoring the active system. The reference wet FPM weight is compared to mud samples collected from the suction pit of the active system as the well progresses. The difference in the weight observations is then used to determine appropriate additions to maintain or exceed the minimum requirements.