Applying Biological Enzyme to Remove Plugging in Screen Pipe Completed Horizontal Wells in Volcanic Gas Reservoir
Authors: Zhang Yingan , Wang Hongwei , Liu Guangyu , Zhu Zhaopeng , Duan Yongwei , Pang Qiqiang , Zhou Xueping 
Published: May 2016

Abstract
Since the grade I reservoir in Changshen volcanic gas field has high natural deliverability. Horizontal wells completed with screen pipe were applied to develop this type of reservoir. However, the severe mud pollution during drilling badly damaged the formation, which reduced the well deliverability greatly. In order to remove the mud cake near well bore and restore the reservoir permeability, a new plugging relief technology of biological enzyme was developed. According to the situation of screen completed horizontal wells and the degradation characteristics of the enzyme, laboratory experiments and field applications were conducted. The results show that biological enzyme can remove the reservoir pollution and recover the natural production of screen completed horizontal wells in Jilin volcanic gas field. It provided a new way to develop the grade I reservoir in Changshen volcanic rock gas field.

Introduction
Changshen gas field is located in the center of the Changling fault depression which lies in the south of Songliao basin. As it kept uplift status during the Yingcheng Sedimentary period, its formation was controlled by dual mechanisms of volcanic activity and tectonic movement. In September 2005, well Changshen X was drilled in this area, which showed that the Yingcheng volcanic layer group has a long gassy well section and that the physical property and gas abundance was good near the crater. The drill stem test(DST) got such a high production as 46×104m3/d in this well. Following evaluation wells, control wells and exploration wells all received success. These indicate that the exploration and development of Changling faulted structure has a good prospect.
Screen completed horizontal wells was applied to develop the grade I volcanic gas reservoir of Yingcheng layer group. The total measure depth of well CS- X was 4360.41m. Volcanic reservoir layer was discovered while drilling to 3616 m. gas survey showed that gas bearing formation became good in 3669 m. The main lithology of the reservoir was in-situ dissolution breccia and stomatal rhyolite. The reservoir types were fracture, pore or pore-fracture. The core analysis data showed that the reservoir porosity was 5.3–21.4%, average 13.1%. The permeability was 0.09–1.77 mD average 0.565 mD. It was also indicated that the in-situ dissolution breccia was the best reservoir type in Yingcheng volcanic group, which comprise the grade I reservoir. Since it had a high productivity, screen completed horizontal wells were adopted. The well configuration schematic diagram of well CSP1 is shown in figure 1, which has a screen pipe completed section length of 735.3 m.
With intensive fracture development, reservoir invasion by drilling fluid was serious in Changshen gas reservoirs. Thus thick mud cake could form near well bore, which caused reservoir pollution and productivity decrease. This paper developed a bio-enzyme technology to remove plugging of screen completed horizontal well in volcanic gas reservoir. According to the situation of screencompleted horizontal wells and the degradation characteristics of the enzyme, laboratory experiments and field applications were conducted. Both laboratory experiments and field applications received success. Reservoir pollutions were relieved and well deliverability got recovered. It provided a new way to develop the grade I reservoir in Changshen volcanic rock gas field.