Oil and Gas Expertise for Gas Shale Fracturing

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 76 Oil and Gas Expertise for Gas Shale Fracturing Mingxing Bai, Xiaohua Pan, Lingyu Wang, Xue Sun, and Renke Petersen Abstract Unconventional shale gas is especially common right now and the development of unconventional gas has been a frequent topic of discussion lately. They are normally related to ultra-low permeabilities and have to be developed with extra processes like fracturing or multistage fracturing. Over the past decades, the oil and gas sector has developed high level technologies, including stimulation technologies, to explore oil and gas. Since the shale formation has very special characteristics, the fracturing processes are somehow different from traditional fracturing technologies in the oil and gas sector. The oil and gas expertise should be transferred to the shale gas production. In order to better understand the most important yet complex process in developing shale gas, this paper is going to provide an overview of the basic knowledge of oil and gas fracturing with explanations of the important characteristics of fracturing processes and then the focus will be on the explanation of horizontal wells fracturing for shale gas production. The following questions will be discussed throughout the paper: What do fracturing fluids contain? What kind of fracturing processes do exist? What is refracturing? How does multistage fracturing work? How can we evaluate the environmental issues? not only in U.S. but also in some parts of Europe. Shale gas belongs to the category of unconventional natural gas reserves, contained in fine-grained, organic-rich, and sedimentary rocks, including shale, but composed of mud containing minerals like quartz and calcite. The first produced shale gas took place in around 1820 in the USA. Technology has made the production of shale gas widely possible and more conventional in the last few years. Latest improvements have also made it possible to increase the recovery of initial gas-in-place (GIP) from 2% to around 50% [1]. In Fig. 1 we can see the basic characteristics of gas production. The concept of the resource triangle was used by Masters [2] to find a large gas field and build a company in the 1970 s [3]. On one hand it is written that there are large gas resources in shale but on the other hand these resources are difficult to develop. This is because of the low permeability and reservoir quality. But even with these poor characteristics, the technology has greatly improved and more exploration and production wells are drilled every day. The ability to drill horizontal wells makes the production of shale gas much more effective than producing through a vertical well. Index Terms fracturing, multistage fracturing, oil and gas expertise, refracturing, shale gas I. INTRODUCTION Current increasing demand and lagging supply of energy mean high prices for oil and gas. This makes unconventional gas even more valuable to a competitive producer at this time, Mingxing Bai is with the Northeast Petroleum University, , Daqing, China baimingxing@hotmail.com Xiaohua Pan is with No. 9 Oil Production Company of Daqing Oilfield Co., LTD, , Daqing, China. Lingyu Wang is with the Well Testing and Perforating Services Company, , Daqing, China. Xue Sun is with the Daqing Oilfield Chemical Company Limited, , Daqing, China. Renke Peterson is with the Clausthal University of Technology, 38678, Clausthal-Zellerfeld, Germany. Fig. 1. The unconventional gas resource triangle [3] Gas can be stored in three different ways in shale reservoirs [4]: 1. Free Gas a. In the rock matrix pores; b. In nature. 2. Sorbed Gas a. Absorbed (chemically bound) to the organic matter (kerogen) and mineral surfaces within the natural fractures; b. Adsorbed (physically bound) to the organic matter (kerogen) and mineral surfaces within the matrix rock.

2 International Journal of Engineering & Technology IJET-IJENS Vol:12 No: Dissolved in the hydrocarbon liquids present in the bitumen. Almost all shale gas wells produce only dry gas, with only a few exceptions. One of the exceptions is the Eagle Ford, which also produces a little bit of wet gas. The Antrim and New Albany, which are mainly minor players producing formation water. There are a few things we have to learn about shale: No two shales are alike. They vary within a trend even along the wellbore [1]; A minor difficulty is finding the gas, but the major difficulty is finding the best area of production and recovery; There are no optimum completion and stimulation techniques for shale gas wells; Natural matrix permeability of shale is extremely low, often in the nano Darcy range; In most cases, it is difficult to predict the life of the well; The formation has to be fractured; otherwise the hydrocarbon will not flow; Wide range of mineralogy. This paper begins with an overview over the shale gas production/exploration in the world, followed by basic knowledge of oil and gas fracturing with explanations of the important characteristics of fracturing processes. The main focus is on the explanation of horizontal wells fracturing for shale gas production. Since the fracturing technology in the traditional oil and gas sector has improved continuously and is very mature nowadays, it can provide a reference and identify these technical areas where future research effort should be addressed to improve the technology transfer. II. SHALE GAS PRODUCTION IN THE WORLD Until just recently, the production of shale gas was almost exclusive to the U.S. This is because other counties were limited on geological knowledge as well as the higher technical and economical costs. The U.S. raised their production from 1.6% in 1996 to almost 10% of all gas production in They also raised their shale gas reserves from 21.7 tcf (trillion cubic feet) to 32.8 tcf because of high success with the Barnett Shale in Texas in However, Europe has also made some important advances in shale gas production, for example, the Alum Shale in Sweden, the Silurian Shale in Poland and Austria s Mikulov Shale. Shale gas can always be found proximal to conventional gas reservoirs, but similar source rock can also found in parts of the world where no significant conventional gas reservoirs exist. This is why people predict lots of shale gas resources [5]. In Table 1 we can see the results of the 2011 Energy Information Administration study of the total GIP resources of shale gas [6]. This study is from [7] and is the most recent estimation of worldwide unconventional gas resources. It compares the shale gas resources from the U.S. to the rest of the world. TABLE I WORLD SHALE GAS RESOURCES (AFTER [4]) Country 2006 (Tcf) 2011 (Tcf) North America Latin America Western Europe Former Soviet Middle East and North Africa Asia World In Fig. 2 the Technically Recoverable Resources (TRR) for shale gas in some countries are shown. It is an estimation based on the TRR of the individual regions. The total TRR of the countries is about 6622 Tcf. Saudi Arabia, which was not covered in Table 1, is now included and is shown as the fifth largest with 645 Tcf. Fig. 2. Shale gas TRR resources in trillion cubic feet [4] Because there is a lot production of shale gas in the U.S., Fig. 3 shows the gas production since 1990 and predicts the gas production till 2035 in the U.S. In 2012 the shale gas production was already 23% of the total U.S. gas production. Reference [8] predicted that the shale gas production in 2035 will reach 49%, which is half of all production. Fig. 3. U.S. natural gas production from 1990 to 2035 [6] There are many important shale gas resources, e.g., Fayetteville Shale, Marcellus Shale, Haynesville Shale, Woodford Shale and the Barnett Shale. The Barnett Shale is the most popular and successful shale gas until now. It lies in the Fort Worth Basin in north-central Texas and expands

3 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 78 across an area of about 28,000 mile 2. The highest production of Barnett Shale is in the northern part of the basin, because the reservoir is really thick there. The first vertical well drilled into the Barnett Shale was in the early 1980 s. The main shale production play was almost two decades later, when the first horizontal well was drilled in In 2011 there were about 12,561 wells drilled into the Barnett Shale Reservoirs. About 9,449 wells were drilled vertically and 3,112 horizontally. From these wells, 8,270 bcf have been produced, of which 75% is from horizontal wells. In Fig. 4 the extreme increase in the number of horizontal drilled wells can be seen [9]. With a current estimated production of 8 billion cubic feet, Barnett shale gas production has made a majorly successful step in the direction of shale gas production. The economic success of the Barnett production is leading companies to develop more and more shale gas. III. Fig. 4. Barnett Shale production in the U.S. [9] HYDERAULIC FRACTURING IN THE OIL AND GAS SECTOR Natural fractures are necessary for the primary and secondary fracturing process. They have the ability to open and maintain flow, which is the key for the production of shale gas. The main challenge is to locate the natural fractures and optimize their response while fracturing fluids are being injected. The natural fracturing formation systems are very anisotropic packages. The exact knowledge of the natural fracture system also provides the advantage of optimizing the frac design by injecting the exact amount on proppant material at the right place and maximizes the frac development [1]. Hydraulic fracturing is used to increase the production rate of an oil or gas well. Fractures provide a larger connection of the reservoir to the production well. The number of fractured wells has been growing enormously. In 1950 there was one fracture job a day, in 2008 there was an estimated 60 to 70+ jobs per day globally. The fracturing design is required to establish a completion, which allows the best possible connection between the wellbore and the formation. Every fracture design starts in the office, where the first decision has to be made. Will it be a potential investigation or not? An effective application of a fracturing process requires several technologies to work together. It begins with the physical equipment used for preparation, mixing and pumping the fluids into the wellbore. The monitoring and recording equipment is also needed to control the abovementioned operations. Such equipment can be really simple, but can also be very complex. The next requirements are the additives such as the fluids and the proppant, which holds the frac open after the frac is done. Before the fracturing process can start, the data must be closely studied in order to gain an understanding of the specific geology, geomechanics, formation properties, and reservoir properties. It is also beneficial to have some information about the hydraulic fracturing placement in this zone. The results of production from variations of fracture geometry and the effective conductivity are also important. If all data is provided, it can be inputted into the simulation model to clarify the design options, as well as to provide insight into choosing the model that will be most favorable for this special well. The specifics of fracturing include mainly two things [1]: 1. How to start the first injection from each stage: a. How to locate the cluster; b. Achieve minimum stress while still achieving good fracturing results; c. Optimize the number of perforation clusters to get the best design rate; d. Establish a fracture in each cluster during rate ramp up. 2. How to drive the frac outward and create the best length when using pad locations and well patterns, but retaining ability to fill in the areas between the staged fracs with a network of effective and open natural fractures: a. Optimum rate ramp up; b. The best spacing of the offset wells; c. Thickness of net and gross pay; d. Frac boundaries and avoid activities which could cause a breakage of the boundary; e. Data of the location, such as density and so on of the first, second and third natural fracs; f. Geological hazards and boundaries information. The additives in fracturing typically consist of: Acid stage consists of several thousand gallons of water mixed with a dilute acid such as hydrochloric or muriatic; Pad stage consists of 100,000 gallons of slickwater without proppant material; Prop sequence stage consists of water including some proppant material, which holds the space open after fracturing. Proppant material might vary from really small particles to coarser sizes; Flushing stage consists of a volume of fresh water to flush the excess proppant from the wellbore.

4 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 79 Following steps have to be done for a fracturing process: Fig. 5. Fracturing process Barnett Shale production in the U.S. [10] IV. CONSIDERATION FOR FRECTURING GAS SHALE Shale production operations have been on the market for many years, but since around the year 2000, horizontal drilling has become faster, more economical and accurate. With this opportunity the well is able to drain gas from a much larger area than from a single vertical well. There are a lot of case studies on this, for example, Marcellus Shale in Pennsylvania drainage area in cylinder chap: Vertical well: 1,320 ft diameter and 50 high Horizontal well: 2,000-6,000 ft in length and drain a volume of 6,000 ft by 1,320 ft by 50ft thickness. This means the drainage area of a horizontal well is 4000 times greater than the vertical well. This is also a big environmental advantage; because of the large contact surface there is a less need of drilling rigs. Using fewer rigs means less roads, pads and pipelines. It is proven that horizontal wells generate one third less disturbed acres. This means less landscaping and vegetation destroyed, soil erosion and general contraction. Even traffic, noise, dust, and waste, such as used mud, can be reduced. V. FRACTURING FLUID AND PROCEDURE The types of fracturing fluids and the use of these fluids have evolved greatly over the past years. Hydraulic fracturing fluids are used to expand fractures and also to transport the proppant into the fractures so that they do not collapse after fracturing. The proppant is either sand or other granular substances injected into the fractured formation [4]. During the fracturing process, a number of chemicals are injected with the fracturing fluid into the formation. Every component has its own engineering purpose, e.g., some chemicals are injected to reduce the viscosity or the bacterial growth at the reservoir surface. There is a number of different oil- and water- based fluids, which efficiently induce fractures. The components of these fluids vary significantly, from simple water and sand fracturing fluids to complex polymeric substances with a multitude of additives. Each type of these fluids has different characteristics and is specially made for each type of formation. There are some main qualities the fracturing fluid must achieve (Adapted from [11]): Be viscous enough to create a fracture of adequate width; Maximize fluid travel distance to extend fracture length; Be able to transport large amounts of proppant into the fracture; Require minimal gelling agent to allow for easier degradation or breaking and reduced cost. Water has become the most common basis for fracturing fluids; however the fluid can also be based on oil, methanol or their combinations. There are also main fluid categories in which the fracturing fluid can be classified: Gelled fluids, including linear or cross-linked gels; Foamed gels; Plain water and potassium chloride (KCl) water; Acids; Combination treatments (any combination of two or more of the aforementioned fluids). Although it seems that gas and foam fluids would be good fracturing fluids, they are actually worse when compared to slick water fracs (SWF). In 1999 the first Barnett Shale was firstly fractured with SWF and they had 35% better results than the more expansive gel fracs. Slick water fracs have been used in the industry for many years, but they are variable especially in shale formation [12]. SWF has such good results in most shale gas reservoirs because of following matched characteristics: Sand carrying capacity, which is very low in SWF, is less important in brittle shale. Shale can have a lot of micro fractures and lamination. Minimizing the fluid viscosity increases leakoff into these crevices and maximizes the channels for gas flowing.

5 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 80 Increased contact area is produced by very large volumes of water and low matrix permeability that keeps the vast majority of the water in the fractures [13] A slick water fracturing process would include following steps (after [10]): First of all the service companies test their surface equipment, before starting the fracturing process. After the surface equipment is tested, they start to pump an acid treatment to clean the near wellbore area, where mud or cement might have stuck. After this part is done, a little bit of slick water including some friction reducing chemicals will be pumped into the formation. This allows the slickwater to be pumped faster. After the first sack has been pumped, the operators begin the actual fracturing process. Now they are pumping large amounts of slick water combined with small amounts of fine additives, such as sand, into the formation. At the end, once the fracturing process is done, the last flush enters the borehole to remove proppant from the wellbore and the equipment. VI. REFRACTURING Refracturing has been spectacularly successful in shale reservoirs compared to convectional reservoirs. There are two main reasons for this [14]: 1. Initial gel fracs, which were really common at the beginning of shale gas production, were probably damaging the formation, so now refracs add more improvement. 2. Propping of the smaller fractures and micro fractures in shale is not effective production losses over time do not appear totally related to flush production depletion and may be linked to fracture closing, poor cleanup, or poor production practices. Most of the documented refracs took place in the formation where gelled or foam fracturing has been done before. Refracs even improve the production rate of good wells and increase the recovery. VII. MULTISTAGE FRACTURING The advantages of drilling horizontal wells, in combination with the ability of massive multi-stage hydraulic fracturing (MMHF) has changed the natural gas industry worldwide. A lot of gas resources in low permeability strata are being unlocked in the north of the U.S. But MMHF will also have a big impact on the oil industry, such as the Monterey Formation (shale) in California, the Eagle Ford Shale in Texas and also in regions out of the U.S., for example, in Egypt the nonfractured part of the dolomitic heavy oil carbonate, which contains over 2.5 billion barrels of oil. Basically MMHF is expanding the drainage volume around the well as is shown in Fig. 6. To this end, large volume hydraulic fracturing is usually executed at many perforated locations along a cased horizontal well, usually drilled parallel to σ hmin to maximize fracture length extension normal to the well axis [15]. In Fig. 6 the six dilated zones along each well are also called stimulated zone. These zones should normally have an ellipsoid shape with a short axis parallel to the σ 3 direction, but it also depends on the formation geology. Typically the length for this kind of operation is m and is located at the basis of the reservoir, because the fracs tend to rise when fluids are injected. This is caused by the buoyancy effect and occurs as soon as the fracturing fluid pressure gradient is less than the local σ hmin gradient. Today s record is 45 separate fracturing stages along a single well. Fig. 6. Multiple HF Stages along the well axis for shale gas stimulation [15] The direction of the fracture does not matter, but the following points have to be remembered: The economic impact of multistage fracturing is huge. There are millions of dollar of resources involved in one fracturing process. There will be no fracturing without monitoring the process and looking at good quality data to make the best model. Always analyze the formation and do your best modeling job so that the fracturing process after the current one will have good data. ENVIRONMENTAL ISSUES The entire process from exploration till production of unconventional reservoirs can be divided into many phases. During these phases there are many environmental issues to be addressed, e.g., waste water, truck traffic including related impacts and risk, air emissions, noise, visual impacts, chemicals and additives being fractured into the ground formation, potential risk of impacting soil and/or groundwater, surface impacts and vibration [16]. For this, a risk estimation and evaluation needs to be carried out. The risk estimation and evaluation should contain the following: Overview about the characterization of natural gas resources, especially the permeability and significant

6 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 81 water flowing channels that have a large extension in the reservoir; Fundamental principles for exploration and production of unconventional hydrocarbon reservoirs have to be compiled. Also a survey about possible hazards on protected (natural) resources has to be compiled and described; Scientific information about the risks to the environment and especially the drinking water; Geological map with delimitation of exploration and producing areas; The experience of current explorations and production projects and those from the past especially in the USA have to be taken into consideration. For example U.S. started to reduce the number of well pads by installing multiply wells on single well pads, so that the surface impacts and footprints also decreased. They developed intelligent water and waste water management plans, so there is not such a high water waste anymore. They are also monitoring all emissions like air, waste water, noise and so on. Downhole environmental risk to fresh water supplies is close to zero. The U.S. proved that from engineering analysis using the report and documented results from more than 10,000 shale wells, which were at least 500 ft below the water sand, had no leakage to the fresh water sand. In Europe most shale gas formations are located at least at 2500 m depths, while the deepest fresh water supply is at 600 m. Also the migration and management programs used in the U.S. will help the European countries like Poland to start with a much better environmental plan for their production. VIII. CONCLUSIONS Shale gas production has been very common in the U.S., especially production from the Barnett Shale. In this paper, the technologies for shale gas production were identified, which correspond to the high level technologies in the oil and gas industry. The advanced technologies for shale gas production include, for instance, refracturing, multistage fracturing, special fracturing fluid design, and so on. Although the oil and gas expertise fully complements shale gas exploitation, there are still some areas where future efforts should be put on to improve the technology transfer between traditional oil and gas and shale gas sector, for instance, advanced fracture design and modeling, selection of more environmental friendly fracturing fluids and proppant, risk evaluation for local water supply and so on. REFERENCES [1] G. E. King, Thirty years of gas shale fracturing: what have we learned?, SPE , presented at the SPE Annual Technical Conference and Exhibition, Florence, Italy, September [2] J. A. Masters, Deep basin gas trap, Western Canada, AAPG Bulletin 1979, 63(2): 152. [3] S. A. Holditch, Tight gas sands, SPE , Journal of Petroleum Technology, June 2006: [4] R. L. Kennedy, W. N. Knecht, and D. T. 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