5.5.1: Drilling Operations

We have, so far, outlined the exploration, leasing, permitting, and site design for drilling Marcellus Shale wells in Pennsylvania. Now we need to develop an understanding of the drilling process and the protections against environmental impacts that are implemented during that process.

A first step in preparing the surface for the drill hole is to install "structural casing" which is a large diameter conduit that is usually dug or pounded in to shallow depth to protect loose near-surface formations (prevent caving into the hole) and to enable circulation of drilling fluid.

Drilling requires a drill string that can be added in segments (typically 30-feet) as the hole deepens with a durable "bit" (the part that grinds up the rock) at the bottom. The bit must be sufficiently strong and hard because if it fails the entire drill string needs to be "tripped" back to the surface (pulled up and taken apart in 30-foot sections) in order to replace the broken or worn bit with a new one. Bits vary in design, but either a roller-cone bit with three rotating cones or a fixed cutter bit is used commonly. The steel cones or fixed cutting surfaces are studded with harder tungsten carbide or even diamonds to improve abrasion and durability. Quartz, a typical component of sedimentary rocks, and steel have about the same hardness such that an all steel bit would not cut rock effectively. The drill string needs to have considerable strength and is suspended from the rig at the surface (the string is actually in tension rather than compression, otherwise it would likely fail; in other words drilling rock is not like the bit on the end of your wood drill that you apply pressure to so that it penetrates deeper into the wood. In addition, there must be some sort of fluid circulation in the hole 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 jam the drill string); and 3) to counteract the higher potential gas 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 or fresh water or fresh-water mud to prevent contamination of the shallow fresh water 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 (less than 1000 parts per million dissolved solids), to waters that, at least in the Appalachian Basin, 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 "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. The "Driller", who is in charge of the progress of drilling in the hole, sits in the "doghouse" surrounded by instruments that monitor downhole conditions, weight-on-bit, and mud weight, and with a clear view of operations on the rig.

The process of actually drilling a well begins with drilling in and setting the "conductor casing" which is the largest diameter casing in the wellbore. Casing must meet certain strength standards (API, American Petroleum Institute sets these) and is fabricated from strong, low-carbon steel. This casing helps to maintain borehole stability and prevent contamination of surface aquifers and to protect against shallow gas. The conductor casing provides a connection for the installation of the casing head and blowout prevention stack. This casing will be set to some depth (usually 500 to 1000 feet in PA, mandated to extend to 50 feet below the last "fresh" water), and will be cemented from its shoe (bottom flange) to the surface to prevent migration of gas or fluids along the outside of the casing where there is a gap between the casing and the surrounding formation. 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 bit used to drill vertical holes decreases with depth. Casing (hole) diameter decreases with depth in a typical shale gas well. Surface casing is set to provide blowout protection, isolate water sands, and prevent lost circulation (drilling fluids flowing into low-pressure, porous formations). Intermediate casing provides protection from deeper low-pressure zones, if they occur, and hydrocarbon producing zones that are encountered before the target horizon. Production casing is the last string installed, and is set in the horizontal part of the hole (5.5 inch diameter) and is designed to protect production tubing, which has lower crush strength than steel casing or is perforated itself as the conduit for hydrocarbons from the producing zone. Production tubing, essentially a liner, would typically extend from the producing zone to the surface inside the other casing strings.

From the "kick-off" point, a different drilling technique is used, with a special drill collar and "mud motor" that can bend at a maximum angle of 3 degrees. Thus, it takes some vertical distance to actually go from the vertical hole to a horizontal segment, typically about 1000 feet. The mud motor is driven by a special formulation drilling mud pumped at high pressures through the motor system, causing the bit to rotate. However, the speed of rotation is slower (about 50 rpm) than for the vertical drillbit because of the eccentric nature of the angled system, which would cause excessive wear if rotation rates were greater. Interestingly, in this string there is a telemetry system with magnetic sensors that can transmit position information to the surface and a total natural gamma radiation detector that allows "geosteering" through a shale formation. Geosteering means that this drilling system can be operated remotely (possibly from a thousand miles away) and guided by a geologist rather than the "driller" through the zone in the target formation that is deemed most productive of hydrocarbons.

All of the casing strings must be centered in the hole (practically and by regulation). This is accomplished by strategically space "centralizers" that are attached to the outside of the casing. The centralization is critical to the later cement job such that specially formulated cement will ideally fill the "annulus" (the void between the formation and the casing) equally to prevent gaps that could facilitate upward hydrocarbon migration of hydrocarbons to the surface and/or to penetrate shallow horizons and migrate laterally into drinking water sources. Note that poor centralization of casing 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 American Petroleum Institute (API) recommends different cement formulations, depending on downhole conditions (temperature, pressure, presence of sulfide). The cement is mixing and send 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. The cement, by regulation (see DEP Requirements Chapter 78), 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 1200 psi within 72 hours.

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 the degree of fill and bonding to both 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.

The casing and proper cementing constitute the barriers to leakage of hydrocarbons and other fluids to the surface. The main barrier to direct blowout of high-pressure fluids and hydrocarbons from the well during drilling is the "blowout preventer" which, at the top of the casing string, prevents pressured gases from blowing out during drilling operations.