The drilling parameters may be broadly classified under two types:
- Rig and Bit Related Parameters; and
- Formation Parameters.
The rig and bit related parameters can be controlled but the formation parameters have to be dealt with. The following parameters control a drilling activity.
- Pump parameters
- Drillstring properties
- Casing details
- Drilling fluid properties
A brief description of these parameters follows.
WOB: It is the abbreviation for “Weight on Bit.” It represents amount of weight applied onto the bit, which is then transferred to the formation which in turn is the energy created together with string speed that advances the drillstring. This parameter is measured through the drilling line, usually with a strain-gauge, which measures the magnitude of the tension in the line itself, and gives the indirect weight reading based on the calibration. This sensor measures a unique value, which is the overall weight (hook-load) of the string including the weight of the block and Top Drive System (TDS). TDS is a mechanical device on a drilling rig that provides clockwise torque to the drillstring to drill a borehole. It is an alternative to the rotary table and kelly drive. It is located at the swivel’s place below the traveling block and moves vertically up and down the derrick.
RPM: This parameter stands for “rotations per minute.” It represents the rotational speed of the drillstring. With the invention of TDS, the reading is directly linked to the electronics of the unit itself. It is considered that the measurements for this parameter are accurate as long as the acquisition system setup has been thoroughly made up.
Pump parameters: The pump parameters are composed of the liner size in use, pump strokes, and the pump pressure. In case there are two pumps working simultaneously, all of the data for two of the pumps should be acquired. With the electric pumps the stroke is transmitted in the same way as RPM. The pressure at the pump in case of having been acquired could be compared with the reliability of the standpipe pressure. Pump pressure should always be greater than the standpipe pressure. Use of flow meters could also be adapted for accurate flow rate measurements.
Depth: The value of depth. This is indicated by the bit position. The depth is linked to the position of the block, and measured by means of the sensors located at the crown block. Depth is a primary parameter in the drilling process and is in the core of innovations in sensor technologies that have made MWD successful.
Inclination-Azimuth: These two parameters are in the responsibility of the directional driller. However, this is also an important consideration for vertical wells, which invariably show some deviation throughout the drilling process.
ROP: Rate of Penetration (ROP) is the most important parameter, since modern optimization techniques essentially maximize this rate and compare costs associated with different rates. It is measured through the relative change of the position of the block in time. Accurate calibrations are very important in order to have a representative ROP parameter. Factors known to have an effect on rate of penetration are listed under two general classifications such as controllable and environmental. Controllable factors are the factors which can be instantly changed such as weight on bit, bit rotary speed, hydraulics. Environmental factors on the other hand are not controllable such as formation properties, drilling fluid requirements. The reason that drilling fluid is considered to be an environmental factor is due to the fact that a certain amount of density is required in order to obtain certain objectives such as having enough overpressure to avoid flow of formation fluids. Another important factor is the effect of the overall hydraulics to the whole drilling operation which is under the effect of many factors such as lithology, type of the bit, downhole pressure and temperature conditions, drilling parameters, and mainly the rheological properties of the drilling fluid. Rate of penetration performance depends and is a function of the controllable and environmental factors. It has been observed that the drilling rate of 12 penetrations generally increases with decreased Equivalent Circulating Density (ECD). Another important term controlling the rate of penetration is the cuttings transport. Ozbayoglu et al. (2004) conducted extensive sensitivity analysis on cuttings transport for the effects of major drilling parameters, while drilling for horizontal and highly inclined wells. It was concluded that average annular fluid velocity is the dominating parameter on cuttings transport, the higher the flow rate the less the cuttings bed development. Drilling penetration rate and wellbore inclinations beyond 70 did not have any effect on the thickness of the cuttings bed development. Drilling fluid density did have moderate effect on cuttings bed development with a reduction in bed removal with increased viscosities.
String-casing properties: The string and casing properties are very important when the frictional pressure losses are to be calculated. In addition to these hydraulic considerations, string-casing properties are also important from strength of materials perspective.
Fluid properties: Rheological properties and the density of the drilling fluids are also among the very important parameters to be recorded for optimization purposes. Usually the drilling fluid density is measured through calibrated pressure sensors. Rheological properties on the other hand are still measured manually. The need for in situ measurement of rheological properties has been recognized and innovative solutions are forthcoming. This will make the online optimization process more accurate (Cayeux and Daireaux, 2009).
Torque: This parameter is the torque of the drillstring while it is rotating. It is measured by means of TDS systems. Previously the readings for this parameter were relative. This parameter bears greater significance in deviated or horizontal wells than in vertical wells.
MWD: In the late 1960s, the first Measurement While Drilling (MWD) equipment was developed, allowing for drillers to collect real-time data downhole. A present time, MWD offers the fastest growth in innovative technology and plays a central role in real-time optimization of a drilling process.
LWD: LWD stands for Logging While Drilling. Formation related parameters could be captured during drilling and be used in the optimization process. LWD equipment was developed in the 1980s to allow collection of gamma, density, porosity, and resistivity formation measurements. Today, the MWD equipment is able to provide near real-time measurements such as azimuth, inclination, temperature, pressure, revolutions per minute (RPM), downhole torque on bit (TOB), weight on bit (WOB), downhole vibration, and bending moment. Significant advances have been made in this technology (Islam et al., 2018). However, current drilling optimization tools are not yet equipped with proper considerations of LWD.