Synthesis and Application of a Self-Degradable Polymeric Lost Circulation Material

Oil and gas well drilling have long been faced with a technical difficulty-mud loss, and this problem still remains unresolved. Polymer gel lost circulation material (LCM), a kind of LCM commonly used today, has several deficiencies such as low pressure bearing capacity, small expansion after water absorption, poor high temperature and salt resistance as well as inability to self-degrade etc. To deal with these problems, a self-degradable polymer LCM was synthesized with carefully selected monomers at optimum conditions determined by orthogonal experiment. This degradable LCM, when first in contact with water, has low rate of expansion, and is thus easy to enter quickly into the fractures where a plugging layer is formed. The LCM forming the plugging layer continuously swells with time and forms in the fractures an embedded plugging layer, thereby increasing the retention ability of the LCM in the loss zones and the shear strength of the plugging layer. Experimental results show that the saturated water adsorption capacity of this LCM can be as high as 156.9 g/g, and the degradation rate of the LCM after 21 d exceeds 90%. In laboratory experiment, polymer sulfonate drilling fluid samples were treated with 5% of this LCM and several other LCMs presently in use and were then tested on a 3-5 mm wedge-shaped fracture tester, the drilling fluid treated with the new degradable LCM has a pressure bearing capacity of 17.5 MPa, significantly higher than those of the other LCMs. In dynamic damage experiment conducted at simulated formation conditions, rock cores with 3 mm fractures were used to evaluate the ability of the self-degradable LCM to resist formation damage. The experimental results show that the permeability recovery of the cores flooded with the new self-degradable LCM treated drilling fluid was 90%, indicating that the new self-degradable LCM causes very slight formation damage. In field application, the polymer sulfonate drilling fluid treated with the self-degradable LCM is able to successfully plug the fractures through which the mud is lost, proving that the LCM has good application prospects.