Completion Challenges in Unconventional Resources During the Transition Period
Authors: Sadykov Almaz , Taha Syed Muhammad , Mulhim Nayef , Dharwadkar Pavan , Small Bryan , Sohrat Baki , McClelland Kenneth 
Published: November 2018

Initial exploratory wells in the unconventional program utilized existing well menus that have traditionally been successful for conventional wells. As the unconventional program matured, intensifying stage count, stage volumes, and greater pump rates, it became apparent that the standard accepted practices would no longer be valid. This paper addresses the safety and operational challenges that resulted from utilizing a wellhead isolation tool (WHIT) to stimulate unconventional wells completed with 10 k psi wellhead components, and provides lessons learned and internally developed best practices. Without comprehensive maintenance procedures, erosion – within the WHIT and completion hardware at the WHIT's outlet – may develop and propagate. This result would compromise the safety and operational integrity of equipment, potentially culminating in a loss of containment.

Mitigation measures were taken to reduce the metal loss, including: (a) the utilization of the WHIT's high-rate bullnose and blast joint application, (b) controlling pump rate and proppant volume, (c) changes in fracturing fluid type, and (d) addition of degradable fibers with the proppant-laden fluid. The WHIT installation and operational practices were developed to limit the number of the mandrel isolation in the same depth position along the stages. Furthermore, the WHIT was investigated for erosion in its body and valve components by physical measurements. Caliper logs were systematically acquired before and after hydraulic fracture treatment stages to evaluate the areas of erosion below the outlet of the WHIT's mandrel and measure tubing wall thickness loss. Highly accurate radial measurements of the inside diameter variations provided an understanding of the location and severity of mechanical erosion damage generated during the fracturing operation. A result of these investigations and actions was the development and implementation of a comprehensive maintenance plan and tracking system to chart and document utilization and erosion trends.

Based on caliper log results, erosion was observed, resulting from a turbulence within the tool, due to rapid change in velocity during flow progression from the tool's small diameter mandrel to bigger ID tubing. WHIT structural integrity was evaluated after pumping a certain number of stages while recording the volume of proppant, treating pressures and fluid utilization. Metal losses up to 27% near the end of the wellhead isolation tool were revealed in the wells completed with 16 fracturing stages and 2.9 MM lb of proppant at 50 bpm maximum average pumping rate. The tubing size was 5.5″ OD. 4.5″ OD tubing was observed to be susceptible to substantial erosion and its utilization should be considered only in unique circumstances with comprehensive risk mitigation.

Initial solutions implemented included the use of a blast joint, in conjunction with fiber-laden fluid and a WHIT with reciprocating stroke and alternating isolation set points. These resulted in a reduction of the maximum metal losses to 4.9% despite the use of 8.9 MM lb of proppant and a higher maximum average rate of 67 bpm. A long-term solution identified was the utilization of 15 k psi lower master valve, tubing spool, and hanger with a 15 k × 10 k psi adapter spool and a 15 k psi fracturing tree for stimulation operations. After stimulation operations the well head frac equipment was replaced by a 10 k production tree.

The erosive effect of proppant-laden slurries used in multistage proppant fracturing due to the ID change from the use of WHIT had not been previously explored and analyzed. A process was developed to record and compare all elements of the stimulation treatment and the measurements of the completion and critical WHIT components to evaluate potential erosional trends. This process delivered a practical method to evaluate the erosive effect of proppant laden slurries on WHITs and tubing, along with the identification and implementation of mitigation measures. Created methods could be also relevant in conventional applications, where high rates and abrasive slurries are pumped through WHITs in multiple stages.