Drilling Basics-Lecture Notes
Drilling Basics
Measurement fundamentals
Most drilling activities involve measurements. The tests and performance checks that a driller makes involve making and communicating measurements. Measurements may be stated in two ways:
- Absolute terms: In terms that have an exact meaning, which is the same for everyone.
- Relative terms: Measurements and stated sizes are expressed as a comparison, and the meaning is clear only to those who are able to appreciate the significance of the comparison.
The accuracy of absolute measurements varies to suit the feature being measured. For instance, depths may be stated to the nearest 10 mm; measurement of core recovered requires greater accuracy, so it might be stated to the nearest 5 mm; bit diameter might be stated to the nearest 0.1 mm. A driller must know when absolute figures are needed and understand the appropriate degree of accuracy that is required.
Several measurement systems are often encountered in everyday life. Familiar measurement units include length, mass, time, and temperature.
Depth Measurements
Hole depth is the measurement of the first concern for the driller, and there are several methods of measuring hole depth in common use. Two methods give acceptable accuracy:
- Measuring the suspended drill string using tape.
- Measuring by counting and totaling the number of precise lengths of drill rods or pipe – usually in multiples of 3 or 6 m (10 or 20 ft) lengths.
The measurement method using a measuring stick, which was once used for total depth, is now acceptable for interim measurements only. The starting point for all depth measurements is known as hole zero. The position of hole zero must be agreed upon with the drilling supervisor or client, and the adopted position of hole zero is recorded on the driller’s log. Typical positions for hole zero are at the natural surface, at the top of the casing collar, at the top of the rotary table, or at the rock face (underground). Once decided upon, hole zero becomes the starting point for all measurements and may be referred to as the ‘collar level’ or the ‘drilling datum’. As the holes get deeper, depth errors and errors because of heat expansion become more likely. More measurements are involved, and consequently more chances of making mistakes.
The casing is usually measured while it is laying on the rack before being run, and the length of the string of random length casing joints is often decided by selecting appropriate joints.
Shallow water wells, the depth to the water level, and construction holes are measured directly by lowering a weighted tape down the hole to feel the bottom or detect when the tape reaches the water.
Measuring wheels, a method often used on geophysical and other downhole logging machines, are available to measure the length of cable run into a hole.
The actual diameter of the hole is often critical to the driller’s work. Tools have been devised to allow the driller to determine the hole diameter, but it is usually measured by running a caliper log. A driller uses diameter measurements to calculate cement or gravel pack volumes.
Downhole loggers can record and store water level data at prescribed time intervals. This allows the drawdown and recovery to be measured and recorded during the pump test.
Measurement of formation pore pressure is usually done using piezometers. Measurement of pore water pressure or joint water pressure (as opposed to standing water level) has numerous applications in the geotechnical and mining fields.
Other downhole measurements include hole azimuth, critical in mineral exploration. The azimuth and dip at the collar of the hole are measured using surface survey methods.
Borehole fluid measurements are also important. Important aspects are fluid levels and fluid flow. The first thing to find out about a borehole fluid is the pressure it exerts. To learn this, we must know its depth, (i.e. its standing level). We then must find out how the pressure is changing, that is, we must monitor the standing levels and record them at least daily.
Three basic ways of measuring the depth to the water surface are:
- Mechanical: Uses either a water whistle or a plopper on a tape coated with chalk to indicate where the end of the tape has contacted the water.
- Pneumatic: where the air pressure shows the depth of water over the end of the tube.
- Electrical entails the use of an electrical circuit tester to show when the end of the cable contacts the water surface (drawdown meter).
Verticality measurements have their application in the measurement of drifts. To measure drift only, simply measure how far the plumb line or a heavy bailer is displaced from the center of the collar. If a plumb bob is lowered down the hole, the mirror observation method can also be used to assess hole verticality.
An accurate indicator of deviation is obtained by using a heavy bailer on a lightweight bailing line, by measuring hole deviation with the bailer is accomplished by jacking the rig over so that the bailing sheave is over the center of the hole.
Water well drillers often use a mirror to reflect sunlight down the hole to permit a visual check on the straightness of the hole.
Verticality measurements
To measure drift only, simply measure how far the line is displaced from the center of the collar. The actual drift of the plumb bob (and the hole) will be twice the measured displacement because the bob is twice as far from the sheave as the collar is:- Again mark the plumb line at the sheave and lower the new mark to the collar.
- Take readings of line displacement as before.
- The bob is now three times as far from the sheave as the collar is. The drift at this depth is three times the displacement at the collar.
- Repeat the marking of the cable, the lowering, and the measurements to the desired depth.
- If several successive measurements are identical, the plumb line could have touched the inside of the casing
Straightness surveys (water well)
Straightness surveys are similar to alignment surveys, but they do not attempt to measure misalignment. These surveys check that the alignment (straightness) of the hole is better than a defined limit.Fluid levels
Aquifer permeability or development is checked using a flow meter down the hole while water is pumped or added at a known rate higher in the hole.
Water quality may be determined in the laboratory by several methods. On the rig, the quality is checked by measuring the conductivity of water using an electrical conductivity cell or a less sensitive modification of the conductivity cell called a total dissolved solids (TDS) test meter.
Test pumping is carried out by pumping from the bore at a constant rate and at the same time measuring the water level in the pumped bore at prescribed times.
Water Level Measurement Procedures; By use of downhole loggers which can record and store water level data at prescribed time intervals. This allows the drawdown and recovery to be measured and recorded during the pump test.
Drilling Project Cycle
A project life cycle is the sequence of phases that a project goes through from its initiation to its closure. Just like any other project, the drilling process entails a number of phases before a well can be deemed complete and ready for use. In Kenya, the drilling project cycle entails the following steps;
- Borehole siting:
This is usually done by hydrogeologists who make use of a variety of exploration techniques to assess the geophysical properties of the underlying area. This is an investigation of the hydrologic and geologic parameters at the subsurface level in a particular area leading to the formulation of hydrogeological maps, and reports on the data gathered during such a study.
- Permit Application
Before drilling, you need a WRMA Authorization & a NEMA permit. This will take approximately 30 days.
You will have to get a drilling permit from the Water Resources Authority, an Agent of the National Government responsible for regulating the management and use of water resources. Its applications follow the carrying out of a hydrogeological survey.
The NEMA license will also be required. This will be obtained after a registered environmentalist carries out a detailed successful assessment of the impacts of the project on the environment.
Application for a permit from the local authorities
Obtaining a no-objection letter from the water service provider
- Drilling the borehole/borewell
This is the actual cutting/excavation process and its duration depends on the depth of the hole and the formation being drilled. On average, rotary rigs drill 50 meters per day.
- Determination of the borehole yield:
In order to gauge the yield of a water well, an aquifer test is performed. This involves installing a test pump and pumping borehole water for a fixed set of variables; a given time at a given rate, and then assessing the test’s impact on the water level in the borehole. Maximum yield is achieved by increasing the abstraction rate, ensuring optimum drawdown of water in the borehole.
- Borehole equipping:
This can best be done after the well has been completed and the borehole completion report prepared and submitted. This report will guide the determination of the most suitable equipment for the borehole. During installation, the borehole installation team is mobilized and material is transported to the site for installation. Once the pump is installed and connected to the power source, testing is done for an appropriate period and key operation parameters are recorded, after which a detailed operation and maintenance manual is issued. Borehole equipping is divided into four phases: viability study, pump selection & design, installation of the pump, and service test.
Electromechanics
Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems interact with each other. strictly speaking, a manually operated switch is an electromechanical component due to the mechanical movement causing an electrical output. The term is usually understood to refer to devices that involve an electrical signal to create mechanical movement or vice versa. This section will cover engines, transmission systems, hydraulic systems, and power systems, albeit not in detail.
- Power System
The power system on a drilling rig usually consists of a prime mover as the source of raw power and some means to transmit the raw power to the end-use equipment. The prime movers used in the current drilling industry are diesel engines. The hoisting system, in conjunction with the circulating equipment, consumes a major portion of the rig's power. Raw power is transmitted via one of the following systems: mechanical drive, direct current (DC) generator and motor, alternating current (AC), and direct current (DC) motor.
- Transmission system
The rig engine provides power for all rig functions, and this power must be controlled and delivered to the various rig components. Therefore the transmission system; connects the power (i.e. convey it to the desired components), controls the power (i.e. change its speed and torque), and smoothens out power surges and dampens shock loads. Mechanical, hydraulic, and air transmissions are used on medium and small rigs. Mechanical transmissions move power through shafts, gears, chains, and belts. Frequently they incorporate a fluid coupling or torque converter. Sometimes referred to as the ‘transfer case’ or the ‘drive chain’, these transmissions allow drillers to control the power supplied to the job. Most early drilling rigs used a mechanical drive system to transmit power from the engines to the operating equipment such as the draw-works and pumps. The drive system consists of gears, chains, and belts. The system had the following setbacks; shock loading to the engine, inability to produce high torque at low engine rpm which becomes a compounded problem as higher workloads continue to decrease engine RPMs, difficulty in providing low torque output due to minimum engine idle speeds, and gear ratios, and power loss through the gears and chains. Electric drive systems are used in independent wheel drives on large mining haul trucks and in large, electric-powered, and -controlled drilling rigs.
- Transmission connection and disconnection
A clutch provides the means of connecting or disconnecting a drive.
- Hoists and Winches
Winches can be grouped into; manually operated winches, and hydraulic winches. The latter provides variable speeds and high torque.
- Hydraulic System
Hydraulic systems provide the ‘muscle’ and circulatory system in most modern drilling equipment, increasingly replacing mechanical drives. Hydraulics require significant maintenance to operate properly, but if they are protected and properly maintained they are relatively trouble-free. The essentials of the hydraulic system include hydraulic pumps, hydraulic motors, tanks & filters, hoses & fittings, hydraulic valves, accumulators, pulse dampers, coolers, and hydraulic cylinders, among others. More of this information can be found in The Drilling Manual, which can be obtained here 👉 Drilling Manual
Mobilization for a Drilling Project
The Rig Mobilization / Demobilization job allows one to mobilize rig units to make them ready for drilling and completion jobs, and demobilize them when they are no longer needed, after the completion of the task and the attainment of the project objectives. It entails the determination of a suitable campsite, and drilling site, looking for all the required licenses and permits, assembling the required resources (human & financial), risk assessment, and moving the rig, the crew, and the equipment to the site.
Moving the Rig, Crew, and Equipment
The access road is required to transport equipment, crew, and the drilling rig itself to the drilling site and the campsite. During the evaluation of a road access option the following aspects shall be considered:
- Road capacity: The heaviest loads to be transported, maximum axle loads, and the total number of axles expected to pass the road during the operation;
- Safety: Visibility, road width, horizontal and vertical alignment, the requirement for protective structures, overhead obstructions, driving behaviors, signposting, escarpments, distances;
- Climate conditions: Occurring during the period of operations and affecting the quality and long-term serviceability of the road;
- Terrain conditions: Hills, water crossings, load width and headroom, the capacity of bridges, ferry crossings, seasonal inundation, and access for the construction contractor;
- Subsoil conditions: Sand, laterite, black cotton, peat, clay, rock, etc., both from foundation and construction material resource point of view;
- Third-party contractors: Availability, local expertise, materials and equipment, attitude towards safe working and protection of the environment; and
- During the exploration drilling, the use of access roads is for relatively short periods. Expenditure on maintenance should therefore be assessed against capital expenditure on more durable structures. Savings at the expense of safety shall never be made.
Risk Assessment
- Effects on the habitat of animals as a result of a change in the natural drainage, surface water levels, aquifers;
- Effects on fauna and potential hazards to the local population, caused by excavations, filling, erosion, drainage, and motorized traffic;
- Effects of increased accessibility for people by opening up tracks into previously inaccessible areas;
- Finding routes that are less utilized and therefore can be opened again for traffic after completion of the operation without causing resentment of interested communities; and
- Direct impact by the survey team on the environment, including minimizing vegetation cutting and controlling the survey crew (for example, by preventing littering and poaching)
Drill Site and Camp Site
- Minimum rig layout;
- Accessibility of the equipment when the rig mast is laid down;
- Minimum size of the waste pit; and
- Required maneuvering area
- Presence of hazards for example hazardous gas;
- Escape route;
- Accessibility between the sites;
- Company night driving policy;
- Rig company safety policies; and
- Complete risk assessment of the operating area
License and Permits
- Preparation of Environmental Impact Assessment (EIA) report covering the civil engineering, transport, and drilling operations;
- Preparation of a comprehensive Health and Safety plan for the entire work phase, from the start of the survey until demobilization;
- Topographical survey work;
- Obtaining permission to implement the project (for example, from local councils, forestry, building, environmental, irrigation departments, etc.); and
- Land acquisition
Drilling Optimization
Neil Cardy defines drilling optimization refers to improving drilling practices and performance. It is a specialized form of Risk Management. The aim of any drilling optimization or risk management project is to minimize, monitor, and control the probability and/or impact of unfortunate events
Some people in their definition, talk about comparing actual performance against modeled expectations, while others compare to benchmarked performance from offset wells and yet others to trend changes in real-time data. All of these are crucial but as part of the process. The prediction of the drilling rate of penetration (ROP) is one of the key aspects of drilling optimization due to its significant role in reducing expensive drilling costs.
The aim of the optimization process can also vary; often the intention
stated is to increase the instantaneous rate of penetration (ROP), while others take
a broader view and look to improve the overall rate of penetration.
According to Neil, the drilling optimization engineer should look at
the overall picture and identify where there are inefficient operations that
are currently resulting in either non-productive time (NPT) or invisible lost
time (ILT). They
Once identified, cost-effective methods of reducing the NPT and ILT must be devised and agreed upon.
The improvements then need to be examined to see if they worked as expected and
whether there is scope for further improvement. So Drilling Optimisation is a
continuous improvement cycle that is intended to reach the technical limit for
the well.
Drilling optimization is very important during drilling operations, to save time and cost of operation thus increasing the profit.
Is Optimization Important?
Today’s industry challenges are impacting drilling success and overall system cost. Operators are faced with:
- Higher efficiency wellbores result in fewer required wells for reservoir drainage. Fewer wells spaced much farther apart means less offset data for pre-drill risk identification;
- Recognition that traditional pre-drill models are obsolete upon spudding a well due to geology and geomechanics changes from offset to new well;
- Increasingly complex reservoir drilling targets requiring more in-depth knowledge;
- Unknown, but expected governmental regulatory requirements dictating some level of real-time data usage; and
- Continued pressure on existing in-house resources, which will only become more so with activity increases and increasing regulatory requirements.
In order to optimize upcoming wells it is important to understand:
- Problems encountered during previously drilled wells in the same field,
- The effects drilling parameters and equipment specifications have on drilling performance,
- Methods of determining the formation strength or “drillability” encountered in the wellbore, and
- The applications of drilling optimizer in simulating upcoming wells.
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