Drilling Efficiency and Cost for Different Drill Technology in Loose Stratum Ying Chen Doctoral student, Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China e-mail: [email protected] Longchen Duan* Professor, Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China *Corresponding Author, e-mail: [email protected] Yubei Lu Professor of engineering, Henan Engineering Research Center of Deep Exploration, Zhengzhou, 450053, China e-mail: [email protected] Ye Wu Professor, Henan Institute of Engineering, Zhengzhou, 451191, China e-mail: [email protected] ABSTRACT This study aims to analyze the drilling efficiency and cost in loosen stratum. Drilling technology includes mud positive circulation, pump-suction reverse circulation, and air downhole hammer. Drilling efficiency and cost data of 75 drilling projects in loosen stratum in Henan Province, China, were analyzed. Result shows the air downhole hammer has the highest drilling efficiency among drilling technologies in this study, and the average drilling efficiency is 3.76 and 2.17 times compared with mud positive and mud reverse circulation drilling technologies, respectively. The pump-suction reverse circulation drilling technology has the lowest cost in this study. The field study shows air downhole hammer drilling technology can be used in loosen stratum, and the cost still has reducing space. KEYWORDS: Drilling efficiency, drilling cost, mud positive circulation, pump- suction reverse circulation, air downhole hammer INTRODUCTION Drilling technology is widely used in geology exploration and engineering field, such as geotechnical engineering investigation, geological exploration, hydrological well, and water well drilling. Since drilling technology was invented, nowadays different types of drilling technology are being used in the drilling field. Some examples are cable drilling, chilled shot core drilling, mud circulation drilling, reversed circulation rotary drilling, rotary drilling by gas or air circulation, water jetting of steel casing through soft sediments, and so on[1]. Drilling efficiency is related to strata characteristic, drill diameter and depth, drilling technology adaptability with the - 3999 - Vol. 20 [2015], Bund. 9 4000 strata, the rate of downhole accidents, and other human factors. Drilling engineering is a complicated process. One factor change will affect the whole drilling efficiency and economics. For example, the average drilling speed of the loosen strata in Henan Province is around 30 m/d for the depth of more than 1000 m. The most commonly used drilling technologies in loosen strata are mud flush and reversed circulation rotary drilling. At present, air downhole hammer drilling is also tested in loosen strata, and the results are satisfactory. The highest drilling efficiency is up to 87 and 72 m/h in clay and moist clay drilling, respectively [2]. The previous study in geothermal drilling field shows no obvious difference in drilling period to drill a hole diameter of 340 or 142 mm [3]. The previous study shows that the technology development in petroleum drilling has led to cost reductions and creation of new opportunities in deep-water environment[4]. The key point in the research on drilling technology is how to drill fast and safe. Drilling economic analysis and studies, especially different drilling technology comparisons, are seldom conducted. The improvement of drilling technology can increase drilling efficiency and reduce cost, but in engineering practice, the traditional drilling technologies are the first choice, especially in the drilling project with depth less than 300 m. Given the high risk of drilling, only a few people are willing to try new drilling technology. The mud circulation drilling technology is commonly used in loosen strata. Different strata have their own suitable drilling technology, and as the technology improves, the most suitable drilling technology may change. Suitable drilling technology will improve the drilling efficiency and reduce the drilling duration and cost. Three types of drilling technologies are selected in this study: mud positive circulation, mud reverse circulation, and air downhole hammer drilling technologies. The related data were collected from 75 wells that were drilled in 2009–2014. The research area is located in Henan Province, China, and the drilling strata belong to loosen strata. The number of project case data that applied these three types of drilling technology is used to analyze drilling effectiveness and economics for different drilling technologies in loosen strata. DRILLING TECHNOLOGY OVERVIEW Mud Positive Circulation Drilling Technology The weight on bit in mud positive circulation drilling technology is transferred to the drill bit by drill pipes. The drill bit then suffers the downward pressure and rotation effect, and the rock is broken by the drill bit. At the same time, the mud or water along the high-pressure pipe is continuously pumped to the bottom of the drill hole, and circulation works from bottom to top. Enough pump discharge makes the cuttings with the flushing medium return to the ground through the gap between the drill pipe and the drill hole wall. The main equipment for mud positive circulation drilling technology includes drill, mud pump, and other drill tools. During the drilling process, two to five people are required in each team to operate the equipment. Mud Reverse Circulation Drilling Technology The rock-breaking mechanism of mud reverse circulation drilling technology is similar to that of mud positive drilling technology. However, their circulation orientation is opposite. The mud Vol. 20 [2015], Bund. 9 4001 or water injection is continuously fed into the drill hole, and circulation works from bottom to top. The cuttings with the flushing medium return to the ground mud sump through the drill bit, internal of drill pipe, tubes, and centrifugal pump. Based on the different ways that reverse circulation is formed, the mud reverse circulation drilling technology is mainly divided into three categories: pump-suction reverse circulation drilling, jet reverse circulation, and air-lift reverse circulation drilling. The mud reverse circulation drilling technology has the advantages of high efficiency, low drill bit wear, and borehole stability. Its advantages are apparent when it is used in the Quaternary strata construction[5,6]. The main equipment for mud reverse circulation drilling technology includes drill, mud pump, other drill tools, and air compressor. Centrifugal pump is an alternative to form reverse circulation. Three to five people are required to operate the equipment. Air Downhole Hammer Drilling Technology Downhole hammer is located on the bottom of the drill tool assembly. Compressed air enters the downhole hammer through the interior of drill tools. When piston is driven in a reciprocating motion, the function directly affects the drill bit and breaks the rocks. Given that the hammer drives the drill bit on the bottom of the hole, the loss of energy is limited. It is especially suitable for deep hole drilling. The air downhole hammer drilling process affects the rotation at the same time, and the drilling efficiency is higher than the rotation drilling or percussion drilling technology for a similar stratum [7]. Normally, air downhole hammer drilling technology is suitable for hard rock because of the percussion and rotation drilling mechanism. An air downhole hammer drilling experiment is conducted in loosen stratum in this study. The main equipment for air downhole hammer drilling technology includes drill, air compressor, downhole hammer, and other drill tools. Four to five people are required to operate the equipment. FIELD STUDIES Geology Characteristic Four areas located in Henan Province, China, were studied. These areas are Zhengzhou, Hebi, Changge, and Xingyang. Figure 1 shows the location of the four areas. These areas are chosen because drilling projects were conducted for production or living need. The stratum information of the four areas are mainly described as follows: (1) Zhengzhou In Zhengzhou area, the location of the studied drilling projects is near the Yellow River. The formation belongs to fine particle soil, and the drilling process may be jammed by encountered aquifer. For the strata shallower than 100 m, the main aquifer lithologies are medium sand and fine sand, others are argillaceous sandstone, coarse sand and some of gravel. For the well depth from 80 m to 350 m, the strata belongs to middle Pleistocene, lower Pleistocene, and parts of the Tertiary system. The main aquifer lithologies are fine sand, middle fine sand, and a small amount of coarse sand. (2) Hebi In Hebi area, the exposed stratum is simple. It is mostly covered by the Quaternary layers. A few mountain areas have a small amount of exposed Cambrian and Neogene layers. These mountain areas are not in the statistical range of the field study data. During the drilling process in Hebi area, the main stratum lithologies are clay, clay sand, silty sand, fine sand, and medium fine sand. Vol. 20 [2015], Bund. 9 4002 (3) Changge In Changge area, the main stratum lithologies are clay, fine sand, clay fine sand, silty sand, and medium fine sand. (4) Xingyang In Xingyang area, the stratum is mainly covered by the Quaternary layers. The main stratum lithologies are clay, fine sand, medium fine sand, mudstone, shale, and Ordovician limestone. Figure 1: Location of field study areas Field Study Process The