Control Mud Losses into Carbonate Reservoirs with Polymer Gels in Tahe Oilfield

Carbonate reservoirs in the Tahe oilfield are developed with fractures and vugs, drilling into vugs and lost circulation which in turn inducing blowout, borehole wall collapse and stuck pipe have frequently been encountered, and conventional bridging techniques cannot be used to effectively control the mud losses. To deal with the mud loss problem encountered in well workover and completion of old wells, a high temperature polymer gel lost circulation material (LCM) has been developed. Evaluation of the performance of the polymer gel LCM shows that before gelling, the viscosity of the polymer base fluid is in a range of 13 – 14.5 mPa∙s. At 150 ℃, the gelling time of the polymer can be adjusted between 30 min and 150 min. After gelling, the polymer gel loses its mobility and forms a high-strength gel plug. The polymer gel is tested on a 5 cm long iron-made core with 3 mm fractures, which simulates fractures found in the carbonate reservoirs, and a pressure bearing capacity of 2.1 MPa/42 cm is obtained. The polymer gel has good contamination resistance and good stability. The gel of the polymer can be easily broken, by adding alkaline solution into the gel to change the pH value, the gel becomes a low viscosity fluid in 48 h. After gel breaking, the permeability recovery of the rock cores that have been plugged with the gel can be greater than 85%. This high temperature polymer gel can be used to effectively plug formation pores and vugs, it can also be effectively removed away, thus making it a good choice for severe mud losses controlling in workover operations.