Study Of Plugging Mechanism And Technology Of Separating-type Gel Slug

In drilling engineering, plugging technology for vicious lost circulation is one of the "bottlenecks" in oil and gas well drilling technology, which seriously affects the process of exploration and development. And the plugging technology is the technique problem which needs to be solved urgently. At present, the way to plug vicious lost circulation is still mainly relying on the experience plugging method and repeatedly (10 times, dozens or even hundreds of times) plugging to achieve the-purpose of plugging by cumulative effects.Based on analyzing the difficulties of plugging vicious lost circulation and the basic action principles of plugging materials and their technologiesy, Professor Luo proposed the plugging principle of separating-type gel slug. And according to this principle and the requirement of simultaneously satisfying "the necessary conditions to plug the vicious lost circulation in fractured or broken formations", it has developed a a special sealant-special gel ZND by using the-latest theories and research results of "Rheology of Structural Fluid" and "Supramolecular Chemistry". It achieved preliminary effect when the gel ZND was used to deal with vicious lost circulation. However, it was still lack of systematic study of rheology of gel ZND. And the rationality, correctness and feasibility of the separating-type gel slug principle need to be verified yet. The problems must to be resolve or verified include:①whether can the gel ZND satisfy "the necessary conditions to plug the vicious lost circulation in fractured or broken formations"?②whether can the gel ZND form the separating-type gel slug cutting off the possible contact between the formation water and the wellbore fluid, and whether can the separating-type gel slug exit threshold pressure gradient?③the plugging technology based on gel ZND which is not good enough needs further development and improvement, so that it can effectively solve the vicious leakage problems in fractured and broken formations.This paper conducts in-depth theory and experiments and field tests study focused on the issues that must be resolved. The main achievements and recognition include:①Based on the rheology study of gel ZND, it confirmed that the gel ZND had good shear thinning properties, in high-speed ladder (such as 300～500s-1) with lower viscosity (100～200mPa-s),but in low ladder (lower than 10s-1) with a high viscosity (more than 100,000mPa-s, cone plate system test results). Gel can flow easily in the ground pipelines, drill string and the annulus, but flow hard in cracks and pores. And it confirmed that gel ZND had strong'elastic which help gel flooding. And confirmed the gel ZND had static yield stress, which was the necessary condition for separating-type gel slug generating threshold pressure and threshold pressure gradient.②Established a evaluation method to evaluate the difficulty degree of gel ZND been diluted by using dilution heat of gel. The lower the dilution heat, the gel was harder been diluted. And establish sieve method to evaluate the anti-dilution ability of gel under certain time diluted by choosing 20 mesh sieve with 0.9mm aperture as the discrimination means. The results showed gel had strong anti-dilution ability that made the gel fill fractures and pores with its excellent performance, which was the basic condition to form "separating-type gel slug".③Used the PTT constitutive model to describe the nonlinear viscoelastic property of gel. And simplified the situation that flows from narrowest cracks into a wide crack (or flows from pore throat into the channel) to 1:4 sudden expansion channel. Numerically simulated the flow status and swelling properties of gel flow in the 1:4 sudden expansion channel by Polyflow software. Numerical calculations showed that the gel can swell after pass throat. And it could achieve the gel fill the pore of fracture space completely after pass throat or narrow fracture by regulating the Weissenberg number. Gel ZND has the possibility to form separating-type gel slug.④Through experimental study by using developed large pores of formation plugging analog devices and visual fractured plugging analog device separately, it confirmed that the gel ZND could form separating-type gel slug in broken and fracture formations, and the forming separating-type gel slug had threshold pressure gradient which could be regulated. And it verified that the plugging principle of separating-type gel slug had rationality, correctness and feasibility. Gel ZND can realize the principle.⑤Base on the study of the compatibility between the gel ZND and common used drilling fluid systems, and the compatibility between the gel ZND and cement slurry, and the gel properties affected by hydrogen sulfide and carbon dioxide gas, the gel can combine existing types of sealing materials to form integrated application technologies to satisfy the necessary conditions to plug the vicious lost circulation in fractured or broken formations. The integrated application technologies which are based on gel ZND include:A. just use gel ZND for pluggingB. cross-linked gel systemC. gel ZND add other sealing material, such as clay, single letters, bridge agents et al. D. first injectinggel ZND, plugging cement slurry after (first inject gel ZND, then inject plugging cement slurry)⑥The plugging technologies of seprarating-type gel slug were used to deal with vicious lost circulation in more than thirty wells in Changqing oil field and northeast Sichuan area et al which achieved significant effect. Moreover, the technology of first injecting gel ZND, plugging cement slurry after was successfully used to deal with the vicious lost circulation with server blowout in Shuangmiao-1 well and Luojia-2 well caused by negative security window formed by fractured vicious lost circulation and high pressure in three-high gas formation.The plugging technology of separating-type gel slug achieves important technical breakthroughs in vicious lost circulation plugging, which is a new technology developed from new idea.In this paper, it verified the plugging principle of separating-type gel slug has rationality and technical feasibility base on theoretical, experimental and field test study. It can provide field experience, theoretical basis and guidance for field application of the plugging principle and technology of the separating-type gel slug.