Use Gel to Control Severe Mud Losses in Carbonate Reservoir Formations in Tahe Oilfield

Some carbonate reservoirs in Block 10 of the Tahe oilfield are developed with fractures and vugs, and are of high degree of inhomogeneity, resulting in severe mud losses difficult to bring under control with common bridging techniques. Polymeric gels, however, can be used because of their readiness to blocking the loss channels through their own crosslinking strength and sticking of the surfaces of the channels through which the mud is losing. Using partially hydrolyzed polyacrylamide (HPAM), hexamethylenetetramine (HMTA), methyl p-hydroxybenzoate (NIPAGIN) as the core additives, a high temperature gel used for lost circulation control in reservoirs was developed. Laboratory evaluation showed that the apparent viscosity of the gel is in a range of 40 – 45 mPa·s, and time for gelling under 140 ℃ is between 4 h and 10 h. Test of the gel on a fractured steel core (5 cm in length) showed that the gel was able to plug the fractures of 2–5 mm in width, and the pressures across the plugged fractures was 2.5 MPa, and was maintained within 18 d. The gel was broken and flowed back after 22 d of blocking. Efficiency of breaking the gel can be improved with gel breaker; the gel will get broken in 8 h when gel breaker is used. This gel, being able to both efficiently plug fractures and to effectively be removed, shows a broad prospect in controlling severe mud losses in reservoirs.