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空气及天然气钻井【中文5270字】,空气,天然气,钻井,中文
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0【中文 5270 字】空气及天然气钻井第一章：引言此书来源于工程实践，适用于工程师，地球科学家，以及现代旋转钻探的工程技术人员。书中联系天然气钻井技术说明工程计算技术。由于此书涉及多种专业和许多潜在应用，所以作者尽可能地减少使用场方程。为使此书通俗易懂，书中尽量使用简单的术语。书中的大部分，方程都可以用任何一套一致的单位给出。虽然计算例子使用英制，但是读者可以很容易的转换成国际单位制。空气和天然气钻探技术是利用压缩空气或其他气体作为循环回转钻进液，使钻头钻入井底，将岩屑携带到表面。压缩空气或其他气体（如氮气或天然气）能利用自身或淡水，地下水，油等压缩流体注入井底。有三个不同的业务应用这一技术：空气或天然气钻井作业（只用压缩空气或其他气体作为循环液） ，曝气钻井作业（用压缩空气或其他压缩气体混合做压缩流体） ，稳定泡沫钻井作业（用压缩空气或其他气体的压缩形成联系的泡沫液循环） 。1.1 回转钻进回转钻进是在地壳岩层钻深孔的方法。这是一种新的钻探方法，是法国工程师Rudolf Leschot于 1863年首次发明的。该方法最初利用淡水循环液做水井钻机。今天这个方法是唯一凿岩钻深孔技术（大于3000英尺） 。虽然我们不知道空压机最初何时用于钻探水井，但是空气压缩机是在1920年用于钻探石油天然气井的。管道煤气天然气井在1935年利用反循环技术在德州钻凿。今天回转钻进用来钻各种钻孔。多数需钻进岩层的水井和环境监测井都是1利用旋转钻进。在矿业中，回转钻进用来钻先导孔以引导大型骨干钻孔。回转钻进技术用来钻需要通过河流公路及其他自然和人为阻力的水，油，气及其他流体管道。最近，回转钻进钻井正用于光纤通信线路障碍等诸多领域，如工业用地和濒临物种等。回转钻进最尖端的应用是钻深孔钻井用来回收象原油，天然气，地热和水蒸气等资源。流体资源回收井需钻探到规定深度3000尺，高达二万英尺。回转钻进应用机器广泛。鉴于上述，旋转钻探可用来钻探火成岩，变质岩，沉积岩。然而，为回收原油和天然气的深钻钻孔几乎完全在沉积岩中进行。回收热能和水蒸汽的钻孔完全在上述三种岩石类型中进行。旋转钻探方法需要使用破碎或切割岩石的钻头。图11 是一个典型磨齿三牙轮钻头。这种钻头采用了更多的行动推进钻头来破碎岩石（更多细节见第三章） 。这种钻头主要用于沉积岩的钻进。为了提高钻头在岩石中的钻进速度，要求钻头要有一定的轴向力（将钻头压入岩石表面） ，扭矩（轮流各点来克服岩石阻力） ，和能够将影响钻头性能的岩屑带走的冲洗液（见图12 ） 。旋转钻探适用于各种钻机，可以是小型“单一”钻机，或较大的“倍增器”和“三重”钻机。今天大部分陆地旋转钻机采用移动折叠形式。单一钻机有一个接头钻杆垂直桅杆空间。2图13给出了一个典型的单一石油钻机。这中小型钻机具有很高的机动性，主要用来钻浅水井（小于3000尺深度） ，环境监测井，某些采矿井和一些岩土钻孔。小钻井通常都是自推进，图13 这种自推进钻井是 E公司推出的节能明星产品。这些典型钻机使用一个钻杆一个钻颈圈。3单身钻机装有星载空压机或星载泥浆泵。这些钻机能容纳两个子系统。这些钻机在甲板上有一个专用的动力源，或者有一个可以利用卡车发动机做为原动机的启动系统（当卡车静止时）。这些小型钻机通过钻链或电缆驱动下拉系统或液压下来系统提供给钻头轴向力。下拉系统将钻机的一部分重量传递给钻机的顶部直置钻头。钻的头部的扭矩和旋转由可以上下拉动的液压传动系统提供（类似于电力系统用与大型旋转钻机）。许多这些小型单身钻机能够钻与桅杆成45 度和90度之间的井。这些钻机的原动机通常是菜油机。图1 4显示了一个典型的双重钻机。这种钻机也可以自行移动或拖动。图1-5是一个双重自行式钻机的示意。4图1-4中的拖车钻机是乔治E 公司的产品。这种双钻机能钻进大约一万尺深度，通常用于石油和天然气钻井作业，地热钻井作业，深部开采，岩土探矿作业和水井。典型的双重钻机通常采用两个钻颈圈或一个钻杆。这些钻机装有能控制转盘，绞车，泥浆泵的星载原动机，钻头上的轴向力由钻颈圈提供，钻的顶部的旋转和扭矩由转盘提供。双钻机有一个几乎可以折叠起来的撬棍槽或甲板，这就使这些钻机拥有了两钻铤或两钻杆的拉力。这些钻机能使用钻探泥浆（利用星载泥浆泵）或压缩空气或钻探气体（利用外部压缩机）进行钻井作业。这些钻机能达到与桅杆成45 度到90 度间的钻探能力，这些钻机的原动机通常都是柴油燃料，但是很容易就能改建成丙烷或天然气燃料。三重钻机装有多种配置，几乎所有这些钻机都是按部分组装的，这些钻机上的立塔式结构被称做井架。这种小型三重陆地钻机大约可以钻进20000英尺，使用范围是两个钻环和一个钻杆。大型三重钻机用在海上平台，这些钻机的使用范围是三个钻环和一饿钻杆。5图1-5显示了一个典型的自行推进双重钻机。这个钻机装有一个泥浆泵用来钻井泥浆循环。它有一个发动机用来推动钻机在路面上行驶，这样的发动机还可以给没有专用发动机的转盘，绞车，泥浆泵提供能量。象这样的钻机，它的能量转移发动机驱动液压泵将能量转移给液压马达，在传递给转盘，绞车和泥浆泵。桅杆上的钻槽说明了此钻机有一个钻杆两个钻缝。这个钻机利用轮盘提供扭矩给钻的顶部。钻头上的轴向力由钻底部的钻环的重量提供（此钻机没有绳索下拉能力） 。这个例子示意性的显示了一个能提供钻井泥浆和循环水做循环液的钻机。钻机甲板前方的小型空压机是钻机的气动控制装置，然而，这台钻机可以很容易的用来空气和天然气钻井作业。这种类型的钻机（已装有泥浆泵）需要一个辅助构件将外部空气压缩进行空气钻井作业。这种空气钻井作业所需要的压缩系统及相关设备都是有专业的生产商提供的。1.2 循环系统两种循环技术能用于泥浆钻井系统和空气或天然气钻井系统。这两种循环技术是正向6循环和逆向循环。1.2.1 正向循环图1-6是一个回转钻探的例子，是装在典型双向钻机上的正向泥浆循环系统。正向循环要求钻探泥浆（或处理过的水）流过淖泵（泥浆泵） ，穿过桅杆的顶部，穿过回转软管，穿过旋转到达内部，到达钻杆和钻圈的内部，通过钻头（在井的底部）到达钻的外面和钻孔里面的环行空间。钻探泥浆将岩屑带走，经过环形出水面，然后将其带到表面，再用泥浆振动筛将其从泥浆中分离出来；然后泥浆流向泥浆桶（将泥浆吸引到一边再使其流向井中） 。双重（三重）钻机上的污水泵一般都采用正向活塞式。单身钻机的泥浆液通常是在地层表面深挖一个坑，然后垫上不透水的塑料衬垫的淡水。重型的软管从泥浆泵的吸力端（见图1-5）延伸到泥坑中。井水在泥坑中被抽出，通过泵，通过管道系统，通过旋转的软管，通过液压传动系统到达钻杆的内部，然后再通过钻头到达井底。钻井水携带岩屑经由钻杆的外部和钻孔内部的环形面到达表面。在表面带着岩屑的钻探泥浆（水）从环形面流向坑里，岩屑沉积在坑底。单一钻机泵通常都采用小型正向往复活塞式或离心式。7图1-7详细的显示了用在双重或三重钻井的正向流通压缩空气钻井系统。正向循环要求被压缩机压缩的大气被推进桅杆的顶部，通过旋转软管，到达钻杆和钻颈圈的内部，通过钻头（在钻孔的底部）进入到钻与钻孔间的环面空间。压缩空气使岩石破碎并将岩屑带到表面。压缩空气和岩屑经管道进入在表面事先挖好的坑。这些坑铺上防渗塑料衬垫。为了这些存储的钻井泥浆安全的为偏钻井所利用。 沉重的环形流体提供高井底压力要平衡(或过重)高孔储存压力。 8图 1-12:避免损失流通图 1-13 还显示为控制高孔压力繁重的环柱钻井液越有益 ， (增加的箭点向下能力控制高孔隙压力)。 大量的钻井泥浆如何能被控制 。 正如前述太重了钻井泥浆，钻这个正压可导致地层损害。 但正压钻探有一个更大的风险。 如果是太重了钻井泥浆的岩层会结构破裂， 这些可能造成破裂的损失可能导致一个循环泥浆井喷。图 1-13:控制高孔隙压力在过去的十年里，据观察，以流通钻井井底压力稍微有流体的压力之下的9孔隙流体给附近最佳效果。 这种简称为欠平衡钻井钻探。 欠平衡钻井生产对形成如流体钻井无可奈何。 降低或消除这种危险地层损害及排除地层破裂而造成丧失流通。 一般来说，如果孔隙压力存储高改造调整了钻井泥浆重量(添加剂)可以产生适当向欠平衡钻井钻井液。 但是，如果不是异常高孔隙压力随空气和天然气钻井技术要求来减轻钻井液柱的环面。 图 1-14 显示了各种方案的各自的潜力和钻井液地层水保持钻孔。 地层水时常常遇到的一个钻井深度地下目标。 这可以在孔隙水的岩层结构高于目标深度。 如果是用作钻井泥浆循环流体， 压在环泥柱通常足以把流出的水形成岩层裸露于井中。 打火机钻井液井底压力降低，因而在水的压力下，任何暴露或断裂的钻探孔结构和岩层。 图 1-14 显示加重钻井液有更大的能力来应付地层水流入井(箭点向下增加控制地层水)。图 1-14:控制生成水流入1.3.2 流量特性比较材质是钻井泥浆的流动特性和空气钻进很好的例子 。 一个很好的示意性的例子如图 1-15 所表示。 井套管是从水面到 7000 尺，5 月 8 日与空气/直径 8 英寸、28。00 磅/尺，象征性的套管。 出井已经钻套管用 77/8 英寸直10径钻头。 比较在 100 00 英尺的钻是个好榜样 (顶至底)，77/8 英寸口径钻头、500 尺 63/4 英寸外直径 20 英寸内径的十六分之十三 。钻颈圈、9，500 尺的空气41/2 英寸直径 16。60 磅/尺 名义，eus135、数控 50、钻杆。图 1-15: 对照好钻的例子钻井泥浆水力学计算假设进行钻井泥浆重量10 lb/gal (75 lb/ft3)，宾泥产量的10lb/100平方英尺、塑胶粘度centipose30。 钻孔机假设有3个三十二分之十三英寸口径喷嘴、钻井泥浆循环流动比率 300GALS/minute。 图1-16显示预想的压力，在压缩功能的深度作为钻井泥浆。 这个图是在一块内部的压力钻。，压力大约在1，400 psig注在底层内钻孔机弦上方的钻头喷嘴6，000 psig的压力。 又是一个计划11的圆环的压力。 压力约为5，440 psig 排在下方的环形喷嘴钻头、0 psig在高高的环在水面。 图1-16反映了在静水压力重量栏钻井泥浆和流体阻力从内部流程面演练一列和表面的圆环。 这一结果流阻力摩擦压力损失。 总摩擦损失的总和管壁、钻头和阻力孔板流量。 这个例子说明了钻井泥浆钻设计有露天大直径孔板或喷嘴开放该钻头。 这反映了损失大约700 psi防扩散通过钻头。 喷嘴直径较小的比大直径钻头在表面亏损更高的注射压力。图 1-16:钻井泥浆压力与深度空气钻井计算假设进行海上钻井作业的水平。1200scfm能有两个压缩机频率矩阵集，每个所以总容积的流量是2400scfm频率矩阵集钻。假设有3位开口(直径0。80英寸)。 图1-17显示图谋的压力压在了空中功能深入。 这个图是12在一块内部的压力钻，注射压力大约在260 psia在里面钻孔机口弦上方的比特压力为270psia。 又是一个计划的圆环的压力。 末端压力约为260psia排在下方的口和1470比特在结束了在水面热泵线(环面的顶部)。 在钻井泥浆例如 图1-17反映了在静水压力重量栏、压缩空气空气阻力从内部流程面及钻表面的圆环。 这一结果流阻力摩擦压力损失。 这个例子是可压缩流体。 考虑内流钻、 静水的重量栏主宰流量(相对摩擦损失)，此结果注射压力在水面被压在钻的下面 (内钻以上， 钻头开口)。 图 1-17:空气压力与钻探深度图1-18显示气温在压缩功能作为钻井泥浆深度。 这个例子的地温梯度0。01F/ft，地球表层几乎是一个无限热源。钻井泥浆循环系统比压缩空气或其他气体较为显密。因此，作为钻井泥浆流下来了，并通过环钻地表面暖气从岩层透过表面钻孔、通过在钻井泥浆环，透过钢钻的钻井泥浆内。 它假定钻井13泥浆使钻头达到的60F。图 1-18:钻井泥浆温度与深度作为钻井泥浆流下来里面热钻井泥浆流入从温度较高的泥岩层热起来，钻进环。 在井底，井底的泥浆温度达到摄氏160F ，钻井泥浆的流环(通常层流条件)是由激烈的摇滚热平整。在激烈的钻井泥浆流的环外，反过来钻井泥浆流下来钻井。 由于其良好的蓄热能力钻井泥浆环形通道的温度高于气温而且比注射井底温度。 这个例子，钻井泥浆的温度大约是130F。图1-19显示计划的温度在空气可压缩钻井液作为功能深入。 压缩空气钻井液密度比钻井泥浆大幅减少。 因此，压缩空气蓄热素质相对差钻井泥浆低 。 14另外， 压缩空气流入钻井循环系统流入流量急剧动荡，因此钻井内部和环面是剧烈的。 湍流是非常有效率，从表面的热量传递到井中的空气流在环内钻。 假设压缩空气进入钻的顶部是在60F热量迅速转移到热 (或冷)气流井。 在这些条件的压缩空气退出环面以大致相同的空气进入钻的最高气温。图1-19:空气温度与钻探深度图1-19显示温度压缩空气在地热井的任何位置大约都是在这个深度的温度。因此，温度的空气流在内心洞是井底温度160F， 还有一些地方的空气冷却通道的开放，因为它的口钻头在 孔底部。 这是较明显的降温效果，如果用的是钻头喷嘴(当用井下马达)。 这个被称为冷却效应Joule-Thomson效应，可以估计8。 但是现假定这是小型的空气流通，很快返回到井底热温度。图1-20显示计划的具体重量计算钻井泥浆这个例子。 钻井泥浆是可压缩的，因此， 具体重量在流通系统的任何位置都是75 lb/ft3 (or 10 lb/gal)。 由15于温度上升，还有一些轻微的钻井泥浆膨胀。使钻井泥浆流到井底。 影响相对较小，这是忽视这些工程的计算。图1-20:钻井泥浆比重与深度图1-21显示计划的具体重量压缩空气这个例子. 压缩空气注入了钻的顶部在一个特定重量1.3lb/ft3(处于压力260 psia和温度60F). 由于空气流动的压力下的钻仍大致相同. 在底部钻具体重量为1.2lb/ft3(处于压力270psia和温度160F). 压缩空气口入出的钻头底部的环(井底)具体重量1.1lb/ft3(在压力260 psia和温度160 F). 16图1-21:空气钻井深度与具体重量0Air and Gas DrillingChapter One IntroductionThis engineering practice book has been prepared for engineers, earth scientists,and technicians who work in modern rotary drilling operations. The book derives and illustrates engineering calculation techniques associated with air and gas drilling technology. Since this book has been written for a variety of professionals and potential applications, the authors have attempted to minimize the use of field equations. Also the technical terminology used in the book should be easilyunderstood by all those who study this technology. In nearly all parts of the book,equations are presented that can be used with any set of consistent units. Although most of the example calculations use English units, a reader can easily convert to the Systeme Internationale dUnits (SI units) using the tables in Appendix A.Air and gas drilling technology is the utilization of compressed air or other gases as a rotary drilling circulating fluid to carry the rock cuttings to the surface that are generated at the bottom of the well by the advance of the drill bit. The compressed air or other gas (e.g., nitrogen or natural gas) can be used by itself, or can be injected into the well with incompressible fluids such as fresh water,formation water, or formation oil. There are three distinct operational applications for this technology: air or gas drilling operations (using only the compressed air or other gas as the circulating fluid), aerated drilling operations (using compressed air or other gas mixed with an incompressible fluid), and stable foam drilling operations (using the compressed air or other gas with an incompressible fluid to create a continuous foam circulating fluid).11.1 Rotary DrillingRotary drilling is a method used to drill deep boreholes in rock formations of the earths crust. This method is comparatively new, having been first developed by a French civil engineer, Rudolf Leschot, in 1863 3. The method was initially used to drill water wells using fresh water as the circulation fluid. Today this method is the only rock drilling technique used to drill deep boreholes (greater than 3,000 ft).It is not known when air compressors were first used for the drilling of water wells,but it is known that deep petroleum and natural gas wells were drilled utilizing portable air compressors in the 1920s 4. Pipeline gas was used to drill a natural gas well in Texas in 1935 using reverse circulation techniques 5.Today rotary drilling is used to drill a variety of boreholes. Most water wells and environmental monitoring wells drilled into bedrock are constructed using rotary drilling. In the mining industry rotary drilling is used to drill ore body test boreholes and pilot boreholes for guiding larger shaft borings. Rotary drilling techniques are used to drill boreholes for water, oil, gas, and other fluid pipelines that need to pass under rivers, highways, and other natural and man-made obstructions. Most recently, rotary drilling is being used to drill boreholes for fiber optics and other telecommunication lines in obstacle ridden areas such as cites and industrial sites. The most sophisticated application for rotary drilling is the drilling of deep boreholes for the recovery of natural resources such as crude oil, natural gas, and geothermal steam and water. Drilling boreholes for fluid resource recovery requires boreholes drilled to depths of 3,000 ft to as much as 20,000 ft.Rotary drilling is highly versatile. The rotary drilling applications given aboverequire the drilling of igneous, metamorphic, and sedimentary rock. However, the deep drilling of boreholes for the recovery of crude oil and natural gas are almost exclusively carried out in sedimentary rock. Boreholes for the recovery of geothermal 2steam and water are constructed in all three rock types. The rotary drilling method requires the use of a rock cutting or crushing drill bit. Figure 1-1 shows a typical mill tooth tri-cone roller cone bit. This type of drill bit uses more of a crushing action to advance the bit in the rock (see Chapter 3 for more details).This type of bit is used primarily in the drilling of sedimentary rock.To advance the drill bit in rock requires the application of an axial force on the bit (to push the bit into the rock face), torque on the bit (to rotate the bit against the resistance of the rock face), and circulating fluid to clear the rock cuttings away from the bit as the bit generates more cuttings with its advance (see Figure 1-2).3Rotary drilling is carried out with a variety of drilling rigs. These can be small“single” rigs, or larger “double” and “triple” rigs. Today most of the land rotary drilling rigs are mobile units with folding masts. A single drilling rig has a vertical space in its mast for only one joint of drill pipe. A double drilling rig has a vertical space in its mast for two joints of drill pipe and a triple drilling rig space for three joints. Table 1-1 gives the API length ranges for drill collars and drill pipe 6.Figure 1-3 shows a typical single drilling rig. Such small drilling rigs are highly mobile and are used principally to drill shallow (less than 3,000 ft in depth) water wells, environmental monitoring wells, mining related boreholes, and other geotechnical boreholes. These single rigs are usually self-propelled. The selfpropelled drilling rig in Figure 1-3 is a George E. Failing Company Star 30K.These rigs typically use Range 1 drill collars and drill pipe.4Single rigs can be fitted with either an on-board air compressor, or an on-board mud pump. Some of these rigs can accommodate both subsystems. These rigs have either a dedicated prime mover on the rig deck, or have a power-take-off system which allows utilization of the truck motor as a prime mover for the drilling rig equipment (when the truck is stationary). These small drilling rigs provide axial force to the drill bit through the drill string via a chain or cable actuated pull-down system, or hydraulic pull-down system. A pull-down system transfers a portion of the weight of the rig to the top of the drill string and then to the drill bit. The torque and rotation at the top of the drill string is provided by a hydraulic tophead drive (similar to power swivel systems used on larger drilling rigs) which is moved up and down the mast (on a track) by the chain drive pull-down system. Many ofthese small single drilling rigs are capable of drilling with their masts at a 45 angle to the vertical. The prime mover for these rigs is usually diesel fueled.Figure 1-4 shows a typical double drilling rig. Such drilling rigs are also mobile 5and can be self-propelled or trailer mounted. Figure 1-5 shows the schematic of a self-propelled double drilling rig.The trailer mounted drilling rig in Figure 1-4 is a George E. Failing Company SS-40. These double rigs have the capability to drill to depths of approximately 10,000 ft and are used for oil and gas drilling operations, geothermal drilling operations, deep mining and geotechnical drilling operations, and water wells. Double rigs typically use Range 2 drill collars or drill pipe. These rigs are fitted with an on-board prime mover which operates the rotary table, drawworks, and mud pump. The axial force on the drill bit is provided by drill collars. The torque and rotation at the top of the drill string is provided by the kelly and the rotary table.The double drilling rigs have a “crows nest” or “derrick board” nearly midway up the mast. This allows these rigs to pull stands of two drill collar joints or two drill pipe joints. These rigs can carry out drilling operations using drilling mud (with theon-board mud pump) or using compressed air or gas drilling fluids (with external compressors). A few of these drilling rigs are capable of drilling with their masts at a 45 angle to the vertical. The prime mover for these rigs is usually diesel fueled,but 6can easily be converted to propane or natural gas fuels.Triple drilling rigs are available in a variety of configurations. Nearly all of these drilling rigs are assembled and erected from premanufactured sections. The vertical tower structure on these drilling rigs are called derricks. The smaller triple land rigs can drill to approximately 20,000 ft and utilize Range 2 drill collars and drill pipe. Very large triple drilling rigs are used on offshore platforms. These rigs can utilize Range 3 drill collars and drill pipe.The schematic layout in Figure 1-5 shows a typical self-propelled double drilling rig. This example rig is fitted with a mud pump for circulating drilling mud. There is a vehicle engine that is used to propel the rig over the road. The same engine is used in a power-take-off mode to provide power to the rotary table,drawworks, and mud 7pump. For this rig, this power-take-off engine operates a hydraulic pump which provides fluid to hydraulic motors to operate the rotary table,drawworks, and mud pump. The “crows nest” on the mast indicates that the rig is capable of drilling with a stand of two joints of drill pipe. This drilling rig utilizes a rotary table and a kelly to provide torque to the top of the drill string. The axial force on the bit is provided by the weight of the drill collars at the bottom of the drill string (there is no chain pull-down capability for this drilling rig). Thisexample schematic shows a rig with on-board equipment that can provide only drilling mud or treated water as a circulate fluid. The small air compressor at the front of the rig deck is to operate the pneumatic controls of the rig. However, this rig can easily be fitted for air and gas drilling operations. This type of drilling rig(already fitted with a mud pump), would require an auxiliary hook up to external air compressor(s) to carry out an air drilling operation. Such compressor systems andassociated equipment for air drilling operations are usually provided by asubcontractor specializing in these operations.1.2 Circulation SystemsTwo types of circulation techniques can be used for either a mud drilling system or an air or gas drilling system. These are direct circulation and reverse circulation.1.2.1 Direct CirculationFigure 1-6 shows a schematic of a rotary drilling, direct circulation mud system that would be used on a typical double (and triple) drilling rig. Direct circulation requires that the drilling mud (or treated water) flow from the slush pump (or mud pump), through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The drilling mud entrains the 8rock bit cuttings and then flows with the cuttings up the annulus to the surface where the cuttings are removed from the drilling mud by the shale shaker;the drilling mud is returned to the mud tanks (where the slush pump suction side picks up the drilling mud and recirculates the mud back into the well). The slush pumps used on double (and triple) drilling rigs are positive displacement piston typepumps.For single drilling rigs, the drilling fluid is often treated fresh water in a pit dug in the ground surface and lined with an impermeable plastic liner. A heavy duty hose is run from the suction side of the on-board mud pump (see Figure 1-5) to the mud pit. The drilling water is pumped from the pit, through the pump, through an on-board pipe system, through the rotary hose, through the hydraulic tophead drive, down the inside of the drill pipe, and through the drill bit to the bottom of the well.The drilling water entrains the rock cuttings from the advance of the bit and carries the cuttings to the surface via the annulus between the outside of the drill pipe and the inside of the borehole. At the surface the drilling fluid (water) from the annulus with entrained cuttings is returned to the pit where the rock cuttings are allowed to settle out to the bottom. The pumps on single drilling rigs are small positive displacement reciprocating piston or centrifugal type.9Figure 1-7 shows a detailed schematic of a direct circulation compressed air drilling system that would be used on a typical double or triple drilling rig. Direct circulation requires that atmospheric air be compressed by the compressor and then forced through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The compressed air entrains the rock bit cuttings and then flows with the cuttings up the annulus to the surface where the compressed air with the entrained cuttings exit the circulation system via the blooey line. The compressed air and cuttings exit the blooey line into a large pit dug into the ground surface (burn pit). These pits arelined with an impermeable plastic liner.10In order to safely drill boreholes to these deposits heavily weighted drilling muds are utilized. The heavy fluid column in the annulus provides the high bottomhole pressure needed to balance (or overbalance) the high pore pressure of the deposit.Figure 1-13 also shows that the heavier the drilling fluid column in the annulus the more useful the drilling fluid is for controlling high pore pressure (the arrow points downward to increasing capability to control high pore pressure). There are limits to how heavy a drilling mud can be. As was discussed above, too heavy a drilling mud results in overbalanced drilling and this can result in formation damage. But there is a greater risk to overbalanced drilling. If the drilling mud is too heavy the rock formations in the openhole section can fracture. These fracturescould result in a loss of the circulating mud which could result in a blowout.11In the past decade it has been observed that drilling with a circulation fluid that has a bottomhole pressure slightly below that of the pore pressure of the fluid deposit gives near optimum results. This type of drilling is denoted as underbalanced drilling. Underbalanced drilling allows the formation to produce fluid as the drilling progresses. This lowers or eliminates the risk of formation damage and eliminates the possibility of formation fracture and loss of circulation. In general, if the pore pressure of a deposit is high, an engineered adjustment to the drilling mud weight (with additives) can yield the appropriate drilling fluid to assure underbalanced drilling. However, if the pore pressure is not unusually high then air and gas drilling techniques are required to lighten the drilling fluid column in the annulus.Figure 1-14 shows a schematic of the various drilling fluids and their respective potential for keeping formation water out of the drilled borehole. Formation water is often encountered when drilling to a subsurface target depth. This water can be in fracture and pore structures of the rock formations above the target depth. If drilling mud is used as the circulating fluid, the pressure of the mud column in the annulus is usually sufficient to keep formation water from flowing out of the exposed rock formations in the borehole. The lighter drilling fluids have lower bottomhole pressure, thus, the lower the pressure on any water in the exposed fracture or pore structures in the drilled rock formations. Figure 1-14 shows that the heavier drilling fluids have a 12greater ability to cope with formation water flow into to the borehole(the arrow points downward to increasing control of formation water).1.3.2 Flow CharacteristicsA comparison is made of the flow characteristics of mud drilling and air drilling in an example deep well. A schematic of this example well is shown in Figure 1-15. The well is cased from the surface to 7,000 ft with API 8 5/8 inch diameter,28.00 lb/ft nominal, casing. The well has been drilled out of the casing shoe with a 7 7/8 inch diameter drill bit. The comparison is made for drilling at 10,000 ft. The drill string in the example well is made up of (bottom to top), 7 7/8 inch diameterdrill bit, 500 ft of 6 3/4 inch outside diameter by 2 13/16 inch inside diameter drill collars, and 9,500 ft of API 4 1/2 inch diameter, 16.60 lb/ft nominal, EUS135,NC 50, drill pipe.13The mud drilling hydraulics calculations are carried out assuming the drilling mud weight is 10 lb/gal (75 lb/ft3), the Bingham mud yield is 10 lb/100 ft2, and the plastic viscosity is 30 centipose. The drill bit is assumed to have three 13/32 inch diameter nozzles and the drilling mud circulation flow rate is 300 gals/minute. Figure 1-16 shows the plots of the pressures in the incompressible drilling mud as a function of depth. In the figure is a plot of the pressure inside the drill string. The pressure is approximately 1,400 psig at injection and 6,000 psig at the bottom of the inside of the drill string just above the bit nozzles. Also in the figure is a plot of the pressure in the annulus. The pressure is approximately 5,440 psig at the bottom of the annulus just 14below the bit nozzles and 0 psig at the top of the annulus
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