Well Drilling Operations
Once the well pad has been constructed it's time mobilize the drilling rig and assemble it on the well pad to allow well drilling operations to begin. Most energy companies do not own or maintain their own drilling rigs and therefore they lease rigs from a drilling service company. In fact, energy companies generally outsource the majority of the specialty services they need to develop oil and gas, everything from the seismic work, well site development, drilling, and fracturing, but they do provide technical direction, project management and the money to fund the operation. For shale well drilling specialized drilling rigs are required to handle long and heavy drill strings in order to drill thousands of feet into the ground vertically and then thousands more feet horizontally. These rigs may cost $50,000/day or more to lease, along with the costs of drilling the well with a crew and materials such as casing and cement. A typical well in the Marcellus shale may cost $3-5 million to drill and construct, along with another $3-5 million to hydraulically fracture, so a well may cost between $6-10 million dollars to develop before any oil or gas can be produced, on top of all the upfront exploration, leasing, and well pad construction work.
The drilling rigs used in shale development are usually "top-drive" rigs that are capable of "walking" short distances (on the order of 20 feet) in order to drill multiple wells on one well pad without the need to dis-assemble and re-assemble the rig between each well. Drilling rigs have a lot of moving parts and it can be confusing to know what everything does, especially when drillers use funny-sounding names like monkeyboard and doghouse to describe some of the major components. Below is a graphic with the major components of a drilling rig labeled.
Now we need to develop an understanding of the well drilling process and the safety and environmental protections utilized. A first step in preparing the surface for the drill hole is to install structural casing, commonly known as the cellar, which is typically a 6-10' diameter circular excavation or boring that is lined with a corrugated pipe to stabilize near-surface unconsolidated materials (soil) and to provide sufficient working area for drilling equipment. Drilling requires a drilling bit at the end which is attached to drill string that can be added in segments (typically 30-feet) as the borehole is advanced deeper into the earth as the drilling bit chips and grinds the rock. Drilling bits vary in design, but either a roller-cone bit with three rotating cones or a fixed cutter bit depending on geologic conditions. The steel cones or fixed cutting surfaces are studded with harder tungsten carbide or even diamonds to improve cutting, abrasion, and durability. The drill rig needs to have considerable strength to hold and pull thousands of feet of drill pipe and the drilling bit which is suspended from the rig, this is actually in tension rather than compression.
A drilling fluid is necessary to circulate in the borehole around the drill bit for at least three reasons: 1) to cool the bit and provide lubrication; 2) to lift cuttings (the rock fragments formed during drilling) to the surface (otherwise they would clog the hole); and 3) to counteract the higher gas and fluid pressure in deeper horizons that would cause the well to "blow out." In the shallower part of a hole, drilling must be done with air, fresh water or water-based mud to prevent contamination of the shallow freshwater aquifers. Generally, the salt content (salinity) of fluids trapped in small voids (called pores) in sedimentary strata increases with depth from near-surface drinking-water quality groundwater (less than 500 parts per million dissolved solids), to waters that may have nearly 10 times the salt content of ocean water (ocean water averages about 35,000 parts per million dissolved solids). Deeper in the well, the fluids circulating in the hole must be more and more dense so that their weight will counteract the pressure of gases encountered during drilling that will attempt to rise up the hole. These fluids are actually specially-formulated drilling "mud", a mixture of water, clays, and, commonly a dense mineral called "barite." Drillers monitor subsurface pressure while drilling and constantly adjust the mud density to match the pressure to avoid blowouts and other drilling complications. The driller is in charge of advancing the well into the earth, and sits in the "doghouse" (sounds like they may be in trouble, but they're not!) which is a control room that monitors downhole conditions, drilling depth, drilling direction, weight-on-bit, mud weight, and other data with a clear view of operations on the rig floor.
The process of actually drilling a well begins with drilling in and setting the "conductor casing" which is the largest diameter casing (typically 20-24" in diameter) in the wellbore. Casing must meet American Petroleum Institute (API) strength standards and is fabricated from strong, low-carbon steel. The conductor casing helps to maintain borehole stability in soils and weathered bedrock. The conductor casing provides a connection for the installation of the casing head and blowout prevention stack. A "blowout preventer" will be installed on the conductor casing to prevent any higher pressure zones encountered during deeper drilling from causing a loss of well control incident, otherwise known as a blowout. Subsequent strings of casing (surface, intermediate, and production casing) have decreasing diameters and are hung inside the conductor casing to isolate water from producing formations and to control well pressures during drilling and production. The size of the drilling bits used to drill vertical sections of borehole decrease with depth as does the associated casing diameter installed within in each size borehole, some would describe it as telescoping. Surface casing is set to isolate fresh groundwater from the deeper portion of the well, and prevent lost circulation (drilling fluids flowing into low-pressure, porous formations). The intermediate casing provides protection from deeper low-pressure, gas, oil or brine-bearing zones if they are encountered before the target horizon. Production casing is the last string installed and is set from the surface to the end of the horizontal part of the hole (5.5-inch diameter). This figure provides a cross-section of a typical horizontal well, showing typical casing sizes and depths along the with the purpose of each casing string.
From the "kick-off" point, where the well begins the transition from vertical to horizontal, a different drilling technique is used, with a special drill collar and "mud motor" that can bend at a maximum angle of 3 degrees as shown in the figure below. The angle of the drilling bit/mud motor allows the well to be steered in the direction the drilling bit is pointing. As shown above, in the well cross-section, it takes some vertical distance to actually go from the vertical hole to a horizontal segment, typically about 1,000 feet. The mud motor is driven by a specially-formulated drilling mud pumped at high pressures through the mud motor system causing the bit to rotate. However, the speed of rotation is slower (about 50 rpm) than for the vertical drill bit because of the eccentric nature of the angled system, which would cause excessive wear if rotation rates were greater. Just above the drilling bit/mud motor is the "bottom-hole assembly" which houses a telemetry system with magnetic sensors that can transmit position information to the surface sensors near the drilling bit, including gamma radiation detector, gas sensor, resistivity, and density. Getting this information in real time allows the drilling engineering team to steer the drill bit and well borehole through the most desirable sections of the shale, which is known as "geosteering".
The vertical depth and length of a shale well vary depending on geologic conditions. In the Marcellus shale, the vertical section of a well is commonly 5,000-9,000 feet deep while the horizontal section, also known as the lateral, may be several thousand to upwards of 20,000 feet long! The trend in the Marcellus and other shale plays is for longer laterals to be drilled, which intercepts more shale and therefore can make the well more productive.
This video describes the process of drilling a well, installing casing and cementing each section in place prior to the hydraulic fracturing process.
Video: Well pad preparation and drilling in the Marcellus Shale (8:55)
All of the casing strings must be centered in the hole (practically and by regulation). This is accomplished by strategically spaced "centralizers" that are attached to the outside of the casing. Casing centralization is critical to the later cement job such that specially formulated cement will ideally fill the "annulus", which is the gap between the formation and the casing, to prevent upward migration of hydrocarbons to the surface or groundwater. Note that poor centralization of casing and cementing was one of the significant factors in the failure of the BP "Macondo Well" in the Gulf of Mexico in 2010.
Cementing is performed on each string of casing. The API recommends different cement formulations, depending on downhole conditions such as temperature, pressure, etc. The cement is mixed and sent down the inside of the casing, followed by a "wiper plug" that pushes the cement downward, through the casing shoe, and up the outside of the casing (the annulus) towards the surface. The volume of cement used is calculated for the depth of the casing string and the average annulus spacing and, by practice and regulation, cement must appear at the surface to provide assurance that the annulus is filled with cement and sealed. The cement, by regulation, must be allowed a minimum of 8 hours to set, during which there can be no other operations in the well that might disturb the casing, and must reach a required minimum compressive strength of 1,200 psi within 72 hours according to Pennsylvania standards.
There are certain downhole logs that can evaluate the efficacy of cement emplacement. These are generally referred to as "cement bond logs" (CBL) and have the capability of sensing if cement has filled the entire annular space between the casing and formation. The CBLs are sonic logs that are run through the casing strings and can be interpreted in terms of the transmission of sound waves through solids. If there are gaps between cement and either the casing or the formation or fluids present in these gaps certain sound waves will not propagate through them and will not be detected by the logging tool. Such logs are not infallible but are critical to evaluating cement jobs that protect against environmental impacts on the environment—either contamination of fresh-water aquifers or direct emissions of greenhouse gases to the atmosphere.
Once the well has been drilled and constructed with steel casing that has been cemented in place, it is time to begin to "complete" the well with the hydraulic fracturing process, which we will explore in the next section.