Well Integrality Technical Practice of Ultra Deep Ultrahigh Pressure Well in Tarim Oilfield
Authors: Zhang Fuxiang , Yang Xiangtong , Peng Jianxin , Li Ning , Lv Shuanlu , Zeng Nu , Zhang Rixing 
Published: May 2016

There is abundant natural gas in Kuqa foreland area of Tarim basin, it has characteristics of reservoir burial depth (5500–7500m), high gas reservoir pressure (105–125Mpa), complex corrosive medium (the partial pressure of CO2 is beyond 2 Mpa, the chlorinity is between 100000 ppm and 140000ppm), and the overburden lithology of gas reservoir is complex (there is heavy calcium rock and mudstone layer), which bring well integrality high challenge. During over three years practice, through exploring the extreme high pressure down hole string shock checker, tubing tongs torque field proving, establishing extreme high pressure complex corrosion behavior pipe simulation behavior laboratory evaluation criterion, supporting air spider and threaded He gas tight detection device, several techniques are formed initially including extreme high pressure gas well pipe simulation behavior evaluation technique, extreme high pressure gas well wellbore evaluation technique, extreme high pressure gas well string layout and mechanical check technique, completion string quality control technique, it provides dynamic guarantee for long-term secure manufacture in Kuqa extreme high pressure extreme deep gas reservoir.

Introduction

There are tremendous natural gas resources in Quka foreland area of Tarim Basin. But the deep buried depth (5500–7500m TVD), high gas reservoir pressure (105–125Mpa), complex corrosive medium (CO2 partial pressure: 1–4Mpa, chloride ion concentration: 100000–140000ppm) and complex overlying rock lithology all have brought big challenges for well integrity management after being brought into production. According to the international generally accepted definition (as shown in figure 1), gas reservoirs in Quka foreland area of Tarim Basin are typically "Extreme Deep, Extreme High Pressure and High Temperature" with very complex operating conditions. Considering that the low reservoir porosity generally less than10%,low permeability less than 1md and along with some natural fractures developed, most wells have to get better performance after acidizing or hydraulic fracturing. At the initial stage of development, super 13Cr gas seal tubing were used as working string in completion operation. Considering the cost and well control factor, we did the hydraulic fracturing through the same completion string and then put into production directly. Wellbores have already suffered from abnormal high pressure during drilling. Given 115Mpa reservoir pressure, weighed fracturing fluid were used during treatment in most wells and we had 100–120Mpa pumping pressure with 136Mpa as the highest wellhead pressure and 210Mpa bottom of hole treating pressure. Meanwhile, acidizing inhibitor only could limitedly protect super 13Cr tubing under over 150 ?, even tubing/casing corrosion or piercement has happened just during treatment and flowback. Initial statistical result of shows that 35.48% of wells have well integrity problems, 63% of which are tubing string failures, 16% are casing string failures, 21% are wellhead failures and other failures. Workovers have been operated in 5 shut off wells to reproduce, each of which cost 20–40 million RMB. After Analysis above, Tarim Oilfield focus on the tubing string failure as the key well integrity research direction, which cover 63% of all well integrity failures. By fully considering casing program before completion, pressure control during operation, field quality control during tubing trips and practices in some other key parts, the better effectiveness is gained.