Optimum Design Of Cement String String Structure In Shale Gas Horizontal Well

Extraction of shale gas usually uses the bridge plug segment fracturing. The integrality of cement sheath is easily to be destroyed. Then it will influence the isolation of the cement sheath. In order to avoid the fracture fluid channeling from being fracturing segment to been fractured segment, it is necessary to improve the centralizer degree of casing string to protect the cement sheath and enhance the displacement efficiency of the cement paste. The traditional cement technology is that we have to install one centralizer on a casing to improve the centralizer degree as far as possible. And there will be more than200centralizers on the2000m horizontal well. But the casing string downloading is very dangerous because of installing too many centralizers on the casing string. And the centralizer degree did not reach70%because of the fault of the centralizer. So there is a contradiction between centralizer degree and casing string downloading and the isolation is not good when it is fracturing.This paper is the part of the "China Petroleum Innovation Fund"(NO:2013D-5006-0212). This paper researched the law of the centralizer degree influence on the displacement efficiency to find out the appropriate centralizer degree. And the law of centralizer degree, the size of casing and well influence on the cement sheath damage to find out the appropriate centralizer degree and the size of casing and well which can protect the cement sheath. We studied the cement casing string according to the result of the displacement efficiency and the mechanics of cement sheath. Then we researched out the casing string structure and design a new casing centralizer at the same time. At last, design the construction technology according to the new casing string and new centralizer.Studies shown that the cement sheath is not easier been damaged the higher centralizer degree. It also can protect the sheath damaging when the centralizer degree is greater than90%. And the cement sheath which is between61/2in well and41/2in casing is the difficult to be damaged in4cases. The narrow gap is the main plot that it influences the displacement efficiency. The influence of narrow gap on the displacement efficiency is61/2in small when the centralizer degree exceeds90%. It is the highest displacement efficiency which is61/2in borehole and41/2in casing in slim hole. The calculation method of centralizer degree of the new casing string structure was deduced according to mechanics model of the casing string. The new casing string can reduce the centralizer by at least50%on the new casing string and can reduce the risk of casing downloading. We designed a new centralizer whose external diameter is same to the external diameter of the coupling. The centralizer degree of the new centralizer can be100%. The centralizer can enhance the safety of casing downloading and improve the centralizer degree.The new casing string can solve the contradiction between centralizer degree and casing string downloading at the same time. It can enhance the displacement efficiency of the narrow gap. It can solve the problem of fracture fluid channeling because of the problem of the casing string. We applied for an invention patent according to the new casing string (Patent Number:201210334796.5). We developed the software of displacement efficiency analysis. The designed centralizer can fulfill the command of the new casing string. We applied and been authorized two piece of patent (Patent Number:201320021932.5,2012204694039.5). The optimized cement casing string structure in shale gas horizontal well can provides a new choice for cement in shale gas horizontal well.