SPE煤层气英文翻译.doc

1、毕业设计（论文）外文翻译学生姓名：田晓学 号：xxx 专业班级：石油工程指导教师：王明教授 2012年6月10日原文： tate of the Art in Coalbed Methane Drilling Fluids现代煤层气钻井液工艺水平Len V. Baltoiu ,Brent K. Warren, QMax Solutions Inc,Thanos A. Natras,Encana CorpAbstractThe production of methane from wet coalbeds is often associated with the production of sig

2、nificant amounts of water. While producing water is necessary in order to desorb the methane from the coal itself, the damage from the drilling fluids used is difficult to assess because the gas production follows weeks to months after the well is drilled. Commonly asked questions include: What are

3、the important parameters for drilling an organic reservoir rock that is both the source and the trap for the methane? Has the drilling fluid affected the gas production? Are the cleats plugged? Does the “filtercake” have an impact on the flow of water and gas? Are stimulation techniques compatible w

4、ith the drilling fluids used?This paper describes the development of a unique drilling fluid to drill coalbed methane wells, with a special emphasis on horizontal applications. The fluid design incorporates products to match the delicate surface chemistry on the coal, a matting system to provide bot

5、h borehole stability and minimize fluid losses to the cleats, and a BREAKER method of removing the matting system once drilling is completed.Field results from three horizontal wells will be discussed, two of which were drilled with the new drilling fluid system. The wells have demonstrated exceptio

6、nal stability in coal for lengths to 1000 m, controlled drilling rates and ease of running slotted liners. Methods for, and results of, placing the BREAKER in the horizontal wells are covered in depth.IntroductionProduction of methane from coal has become one of the more interesting practices in rec

7、ent years to produce hydrocarbons.1-6 In the United States in 2005, it is estimated that 11.7% of all gas produced is from coalbed methane (CBM) sources7.While in conventional drilling in sandstones and carbonates it is often simple to tell if a drilling fluid is fully or partially responsible for f

8、ormation impairment, this is often much more difficult in CBM wells. When a CBM well depends upon the production of water to reduce formation pressure and thus lead to gas desorption, the influence of drilling fluid becomes masked or even forgotten.As the frontiers of CBM wells are pushed into the h

9、orizontal drilling realm, the importance of the drilling fluid is magnified. The fluid needs to both stabilize the wellbore during the drilling phase, but at the same time needs to minimize any production shortfalls due to damage. A simple N2 fracture which may be used on a 5-10 meter vertical coal

10、seam is not a simple matter to transfer to a 500-1000 meter horizontally drilled coal section.This paper discusses how coal geology impacts drilling planning, drilling practices, the choice of drilling fluid and completion/stimulation techniques for Upper Cretaceous Mannville type coals drilled with

11、in the Western Canadian Sedimentary Basin. A focus on horizontal CBM wells is presented.Basic Coal GeologyIf you were to hand a piece of coal to someone you knew and asked them to describe it, they would probably say “its black”. The fact is they are right, but there is more to coal than what meets

12、the eye, especially if you dont know what youre looking for. Coal is a very complicated organic rock made up of tiny microscopic constituents called macerals, which are analogous to the minerals found in inorganic rocks such as quartz. Macerals are made up of various lithified plant debris such as s

13、pores, resins, pollens, waxes, cuticles, and resins. There are three main groups in which macerals are classified, the vitrinite group, inertinite group, and the liptinite group, all which can be broken down further in to several individual macerals. Macerals of similar character can be grouped into

14、 what are called microlithotypes, microscopically discernible units analogous to laminations in sedimentary rocks such as sandstones. Microlithotypes are further combined to form macroscopically visible units called lithotypes. Lithotypes, which are analogous to beds in other sedimentary rocks, are

15、classified on the basis of their brightness. For example, a lithotype predominantly made up of the vitrinite group would look very bright. In contrast, coal rich in inertinite would look very dull. The dull and bright bands tell us something about the heterogeneity of the coal and how variable its p

16、hysical make up is both vertically and laterally. Each lithotype comes with its own set of physical properties which can enhance or impede production of coalbed methane.Coal rank is another important physical property. Coal rank is a measure of the degree of chemical alteration the coal has undergon

17、e (also referred to as diagenesis). The longer the coalification process goes on, the higher rank the coal becomes. Vitrinite content also changes with coal rank, as do several other physical properties important to CBM potential8 (Figure 1).Figure 1 Coal Ranking Classification by Vitrinite Reflecta

18、nce, Volatile Matter, Bed Moisture and Caloric ValueCleatingThe single most important physical characteristic of any coal relative to CBM production is permeability. Permeability in coal is a direct function of the cleat and/or fracture network present. As coal matures through the process of coalifi

19、cation, moisture and volatile gasses are slowly driven off resulting in the shrinking of the coal matrix. As the coal shrinks, cleats begin to form, similar to the cracks that develop when mud dries under the heat of the sun. The cleat/fracture system in coal is also referred to as the macropore sys

20、tem.A good metaphor for visualization is a loaf of sliced bread (Figure 2). The spaces between the slices of coal are fractures that are referred to as “face cleats”. The spaces within the slices of coal are referred to as “butt cleats”. They may, or may not, intersect with the face cleats. When we

21、refer to coal “permeability” we actually refer to the permeability obtained from the fractures network. Face cleats are very important as they are the backbone of coal permeability. Butt cleats may, contribute to such permeability if they intersect the face cleat network. A third set of fractures ma

22、y be found on coals that have been exposed to tectonic stresses. These third sets of fractures are referred to as “tectonic fractures” and are very important to CBM production. Tectonic fractures increase the permeability of the coal by two mechanisms: (a) their own presence and (b) by connecting to

23、 some of the butt cleats previously not part of the fracture network. With the great heterogeneity within a single coal network, the range of fractures presence can vary from no fractures to complete three-set fracture development.Figure 2 Stylized depiction of a coal showing the face cleats , butt

24、cleats and tectonic fractures Maceral type and coal rank are the two most important controlling factors in cleat development. For example, a vitrinite rich high ranking coal will have excellent potential for cleat development. In contrast an inertinite rich, low rank coal will have very little poten

25、tial for cleat development. As discussed before, coals can be very heterogeneous with several lithotypes over a very small vertical distance (cm to m). If you had two lithotypes vertically adjacent to one another, one rich in vitrinite and the other rich in inertinite, the vitrinite rich layer, or l

26、ithotype, would show better cleating even though the entire coal is of equal rank 9(Figure 3).Figure 3 Outcrop of the Horsehoe Canyon formation, Paintearth Mine, Alta. A thicker, poorly cleated inertinite rich coal overlying a thinner seam of well cleated vitrinite rich coal.Depth is an important fa

27、ctor to consider when talking about cleats and their apertures. Generally speaking, the effective permeability of coal decreases with burial due to compaction.Storage and DesorptionMethane can be stored in two places in coal; in the cleats/fractures, or macropore structure; and in the matrix, or mic

28、ropore structure. The macropore system is where the effective permeability is measured and are the conduits in which the methane travels into the wellbore. The cleats/fractures are filled with methane molecules that have attached, or adsorbed, on the surface of the coal; this is known as “free” gas

29、(Figure 4). In the micropore system, there is significantly less permeability. Imagine a sponge with millions of tiny cavities, but none of them connected to one another. The majority of the methane gas is stored (adsorbed) in these micropores, or matrix porosity, and is referred to as “bound” or “t

30、rapped” methane.Figure 4: Methane molecules adsorbed onto cleat network of coal.Desorption, the opposite of adsorption, occurs when a methane molecule detaches from the cleat face and starts to flow toward an area of lower pressure, in this case the well bore. As desorption continues to take place,

31、the methane adsorbed onto the cleat face, or “free” gas, begins to dissipate. When there is no more “free” gas to be desorbed, the bound or trapped gas will start to make its way into the cleat network by the process of diffusion.10 Once the “bound” methane molecules reach the cleat face, migration

32、to the well bore is accelerated due to the greater permeability of the cleat network.Coal vs. Non-coal ReservoirsThe main characteristic that distinguishes coal bed reservoirs from conventional reservoirs (i.e. sandstones & carbonates) is the source of the hydrocarbons found in them. In conventional

33、 reservoirs, the hydrocarbons are generated in source rocks found elsewhere in the stratigraphic section and over time, migrate into these porous reservoirs and become trapped. Coal bed reservoirs can also become charged with hydrocarbons via migration from organic rich source rocks but, the differe

34、nce is that coal can typically generate its own hydrocarbon from within. No outside source is necessary as coal itself is an organic rich source rock.A second difference is the surface of the coal is more chemically charged than the surface of conventional sandstone or carbonate rocks. What is even

35、more intriguing is the fact that its charge has the tendency to change with the change of the pH in its environment. As seen further in this paper, the surface charge plays an important role in drilling, formation damage and production.Rock MechanicsUnlike most conventional reservoir rocks, coal has

36、 low integrity and is very friable. The Mannville coal in the subsurface of Alberta, Canada, can be found at depths that vary between 600 m to greater than 2000 m. If we consider a cube of rock and its stress distribution, v the vertical stress, Hz max the maximum horizontal stress and Hz min the mi

37、nimum horizontal stress, in coal found at depth, the v is the largest. Coal has a Poissons ratio as much as one order of magnitude higher than conventional rock, and will have the tendency to compress under sufficient load. This phenomenon contributes to borehole instability mechanism when drilling

38、in coal (Figure 5).In conventional reservoir rocks, the Hz max can be as high as six times the Hz min. Therefore, the path of least impact when drilling horizontally is perpendicular to the maximum horizontal stress. In coal the Hz max typically is only 1.6 times as much as the Hz min. However, when

39、 combined with coals low strength, it stands to logic that the same principle applies. Field applications have proved this to be true especially when tectonic fractures were present in coal.11 Not surprisingly, with coal being such a heterogeneous material, exceptions to the guideline above do occur

40、. A narrow corridor between Barrhead and Fort Assiniboine (just northeast of Edmonton in central Alberta) exists where differently orientated horizontal wells have successfully produced coal bed methane.Figure 5: Depiction of stresses and orientations Vertical stress is shown in blue, the maximum ho

41、rizontal stress in red, minimum horizontal stress in green.Drilling ConsiderationsEarly drilling of horizontal wells in the Mannville coal in Alberta encountered tremendous borehole stability problems in most areas. However, innovation and experimentation had led to a three point system that proved

42、quite successful. This system involves careful consideration of: (a) well trajectory/well design; (b) drilling practices and (c) drilling fluid.Well TrajectorySince coal does not have a porosity and permeability in the conventional sense, the question of expected enhanced production from a horizonta

43、l well, as compared to a vertical well is ongoing. While high-density spacing of vertical wells followed by nitrogen fracturing of the coal has proved to be economical in shallow coals such as the Horseshoe Canyon in Alberta6, deeper coal horizons such as the Mannville have proven to be more challen

44、ging. Poor production rates led to economics which do not support high density vertical well drilling for Mannville CBM wells. Essentially, the exposure to the desirable coal cleating network is very limited in vertical wells. Horizontal wells were therefore designed and drilled in order to intersec

45、t as much fracture network as possible, especially the face cleats in thicker Mannville coal seams.In Alberta, due to the presence of the Rocky Mountains arch, the Hz max has a NE-SW orientation. Interestingly enough, the orientation of face cleats runs on the same NE-SW direction. Therefore the wel

46、ls that have recorded better success were designed with trajectories NW-SE or SE-NW, achieving at the same time a near perpendicularity to the maximum horizontal stress and to the face cleats (Figure 6).Figure 6: Map of Alberta showing stresses and cleating. The blue lines represent the preferred or

47、ientation of the face cleats, in a NE-SW direction. This is the same direction as the principle horizontal stress Hz max. Curved red lines indicate the front line of the Rocky Mountains. The majority of the Mannville horizontal CBM wells have been drilled basically NW/SE direction. Practice has show

48、n that coal will not support tight well radius, dog legs or key seating. Therefore low build angles, large radius wells with very smooth curves produced significant benefits on drilling stable wellbores in coal.Well DesignTo date in Alberta, Canada, two distinct well profiles have been utilized: the

49、 “motherbore” design and the “classic” design. The motherbore design involves drilling a deviated intermediate hole through the coal, setting an intermediate casing, cutting a window and drilling multiple horizontal legs into the coal seam (Figure 7). A slotted liner is then run in each leg. The perceived advantages of this design relate to the possibility of drilling multiple ho