Lithology has a varied effect on the core recovery and overall core-drilling program. Photo courtesy of Acero Martin Exploration Inc.

A close coordination between drilling engineers and geologists is very useful in a core-drilling program to complete the work in a cost efficient way. The main objective of coring is to get the complete samples of rock and minerals within optimum cost, which can be reliably and confidently utilized for the further analysis to determine the geological, stratigraphical, structural, geo-engineering and other characteristics of the rocks and minerals of the region under investigation.

The lack of geological understanding may have a high impact on the core drilling time and economics of mineral, geotechnical and hydrocarbon exploration drilling project.

The core exploration drilling experience shows that utilizing and integrating geological information, while designing a drilling system and program is useful to optimize drilling and get the desired core recovery to be used in core testing, reserve estimation, mine planning, and design and development in the mineral and mining industry.

In hydrocarbon industry, core drilling is carried out in prospecting and exploration wells, or when needed to get the more reliable information of source, reservoir rocks or even seal and trap rocks. The geotechnical information also is having utility in hydrocarbon industry, though under-estimated and less utilized. Hydrocarbon reservoirs are in a natural pressure and stress condition before the production. The production of hydrocarbons from the subsurface changes these conditions, resulting in the decrease of porosity due to compaction by the release of reservoir fluid pressure and changes in stress conditions after drilling. To study and analyze this core drilling is a useful tool in the hydrocarbon industry, because the subsurface rock mass characteristics can be more reliably determined from the cores rather than seismic or any other methods because of the limitation of resolution. This information from cores may be correlated with well log and seismic data to extend and model the non-cored zones, formations and regions.

The geo-mechanical changes in hydrocarbon reservoirs are on a comparatively finer scale than the changes in any underground mine development due to the scale of changes in the stress conditions. The hydrocarbon reservoirs development involves the removal of fluids (oil and/gas and water), while the development of underground mines involves removal of rocks from subsurface to make passage to carry out mining of minerals and rocks.

Geological Control and Its Techniques

Figure 2 shows the various geological components that influence the design and operation of core drilling program. Among the various discussed geological components, many are interdependent. Each geological component has been discussed separately, because of their dominance on another geological component at some level.

The stratigraphy provides information about the occurrence, thickness and depth of the various lithological units and their physical, structural, mechanical and chemical characteristics. The stratigraphic information provides the drilling crew a predictive tool. This information related to the presence of particular rocks having specific thickness at specific depth is very useful for designing a drilling program and optimizing the drilling operation, taking into account the efficiency with time and cost. Figure 3 shows a hypothetical stratigraphic section of a particular region. It shows that the region contains geological formations like soil, sandstone, shale and coal with their known thickness, depth of occurrence of each rock, apart from information on the structural disposition of rocks. With this information, we can design the drilling program with more reliability and efficiency. This hypothetical stratigraphic information of a particular region, under investigation by core drilling, shows that a drill rig that can drill down to 2,700 feet is needed. This can be determined by the rotational torque capacity and the hoist capacity of the drilling machines.      

The lithology has a varied effect on the core recovery and overall core-drilling program. The lithological information indicates rock types with a specific hardness and uniaxial compressive strength in general. Integrating this information with the stratigraphy that is depth of occurrence and thickness, it is helpful to design and control the drilling operations efficiently. This also is useful in suitable bit selection for coring, taking into account the thickness of the rocks. If rock has good thickness, then appropriate bit selection is useful for efficiency and economics.

The rock mechanics can be defined as the science of mechanical behavior of rocks under natural as well as artificial stress conditions. When drilling is carried out, the drill bit, through drill strings and drill machine, exerts a pressure on the geological formations. The hardness and uniaxial compressive strength directly affect the drilling penetration rate and core recovery. The higher hardness and uniaxial compressive strength rocks like quartzite have a relatively lower penetration rate and lower drilling rate, but good core recovery in general. The lower hardness and uniaxial compressive strength rocks – like weathered rocks, shale, limestone and sandstone – have relatively higher drilling penetration rate, but poor core recovery in general.

Figure 4 shows the illustration of some structural features of rocks. These structural stratigraphic components like bedding planes, alternate sand/shale sequence, rock cleavages, foliations, anisotropy of the rock mass, joints, cracks and fractures – and their spacing, orientation and frequency – affect the drilling, especially the orientation of the drill direction in the borehole. Many times in core drilling, the tools are more sensitive to follow such planes (joints, anisotropy of the rock mass, fractures, foliations, etc.) where the drilling tool finds less resistance, and thus, deviating the borehole. The deviated drilling tools do not have much applicability, because of the incorrect stratigraphic representative core samples produced. The jointed, fractured and permeable subsurface formation can result in the loss of the borehole circulation. The loss of drilling fluid in the subsurface directly affects the borehole stability, borehole cleaning, rate of penetration, and the performance of the drilling bit. The lack of remediation also can cause the burning of the bit, due to generation of the frictional heat and lack of sufficient amount of the drilling fluid for cooling the bit in the borehole. It increases cost and time. Thus, structural information further helps in designing the core-drilling program and improving the core recovery, while maintaining the drill path in the desired direction.

Inclined sedimentary rocks, due to their anisotropic physical properties by virtue of their deposition in a varied time period, influence core-drilling operations. Horizontally deposited rocks, due to tectonic causes and their magnitude and nature, change their spatial orientation over time. These anisotropic rocks interact with the drill bit surface differently, depending upon the magnitude of the anisotropy affecting core recovery, as well as drilling rate.

Ground water has a profound influence on the core drilling. Figure 5 illustrates the ground water control on core drilling. Sudden ingress of ground water from the subsurface formations can cause washing of the cores, resulting in poor core recovery, as well as blowout in highly overpressured formation zones. Thus, ground water information is helpful in desired core drilling program and operation.

Each rock mass has typical characteristics influencing core recovery. The rock mass composition contains the intact rock characteristics, texture and grain size, joint characteristics, density and pattern of joints. When the rock grain gets finer and the texture is denser, the rate of drilling and rock mass drillability decreases. At the same time, finer-grained and dense-textured rocks give good core recovery. Similarly, a completely intact rock with fewer joints has relatively higher core recovery, but lower drilling penetration rate. In such conditions, use of polymers, which provide lubrication, is useful to increase the rate of drilling and to decrease wear and tear of bits. The coarser grain size in rocks, like coarse-grained sandstone and granites (a coarse-grained igneous rock), has an abrasive effect on the drill bit diamonds – damaging the bit-face diamonds and decreasing drill penetration rates. The joint characteristics, density and pattern also have important influence on drilling tool deviation. The size of diamonds selected for a given type of coring drill bit depends primarily upon the rock formations and the drilling pressure to which the bit will be exposed. The drill bits set with large diamonds are used for core drilling in coarse-grained and sedimentary rocks. The harder and finer-grained the formations, the smaller the diamonds must be used in core drilling. The core-drill bits impregnated with diamond dust are preferred for drilling in very hard, abrasive, compact-grained types of rocks. The rock with higher hardness and uniaxial compressive strength requires increased weight on bit. The dimensions of the bit are determined by the requirements of the drilling program. Polycrystalline diamond bits (PDC) now are commonly used for core drilling in the hydrocarbon industry (including coal bed methane) because of their higher penetration rates in formations like shale, limestone and soft sandstone. The use of PDC bits for core drilling in mineral exploration is not very common yet because of unavailability of cost-benefit data in comparison to other commonly utilized coring bits (surface-set diamond and impregnated diamond bits).

Weathering has profound influence on any core-drilling program. Weathering decomposes and alters the original geochemical, mineralogical and petrological composition, resulting in changes of the hardness, uniaxial compressive strength and other properties of a rock. For example, core exploration drilling projects in Gondwana coals (which is comprised of sandstone, shale and coal sequences in general) has shown that a fresh and unweathered sandstone gives good core recovery, though the drilling rate is comparatively less, while a weathered sandstone creates problem in complete core recovery, though the drilling rate is comparatively higher than a fresh and unweathered sandstone. In general, weathering can turn even granite into a very soft, loose and friable rock. In soft, weathered and loose formations, double-tube core-barrels or triple-tube barrels with bottom discharge bits can be used to get the good core recovery. The use of these core barrels allows less washing of the core from the drilling fluid. Apart from these, the drilling speed can be reduced in order to improve core recovery. In addition to this, polymers like polyacrlyamide may be used to increase and maintain the core recovery. The low-solid-content drilling fluid and appropriate cleaning of the borehole also improve the core recovery.

A mineral composing a rock affects the core-drilling rate as well as core recovery. This has been fairly shown by the many rocks composing minerals. The quartzite and chert rock consists of quartz mineral, which has hardness of 7 on the Mohs scale of hardness. These rocks have a relatively lower rate of core drilling than the rocks comprising of soft minerals like gypsum, calcite and dolomite, which have a hardness in the range of 2 to 4 on the Mohs scale. The quartz and cherty minerals, due to their higher hardness and uniaxial compressive strength, provide higher resistance to the drilling tool weight and pressure at the rock/bit interface, decreasing the rate of penetration. On the other hand, experience has shown relatively good core recovery in the quartzitic and cherty rocks, compared to the gypseous shales, calcitic limestones and dolomites. The most probable reason behind it seems to be the distribution of stress imposed by the bit on the rock because of their higher hardness, uniaxial compressive strength and uniform intact rock mass composition. The soft rocks do not have enough hardness and uniaxial compressive strength to equally distribute the stress the bit imposes on them, resulting in the concentration of bit stress on some localized part, while breaking and crushing the softer rocks. This explains the poor core recovery of soft, weathered, jointed and fractured rocks.

Some Suggestions for Core-drilling Programs

The stratigraphic, lithological, rock mechanics, structure, ground water, mineralogy, rock mass composition, and weathering information of the rocks in the region under investigation can be gained from the previous geological and geophysical reports. In the absence of previously studied reports, a geological mapping can be carried out, involving exposed outcrops of the region. If suitable outcrop exposures are not present in the region, then a geophysical program should be designed, taking into consideration the scope and objective of the work, cost, efficacy of the selected geophysical method, and their utility in the designing of the core-drilling program – and overall mineral and geotechnical exploration objectives.

A low-cost option is the electrical resistivity method (vertical electrical sounding, resistivity profiling), which can be used to infer the geological setup of the region. This also is a relatively low-cost option. This method measures the resistivity of the formations, which depends on the type and amount of the fluid present in the formations. Vertical electrical sounding measures the vertical changes in subsurface, giving information about the lithology, depth and thickness, fluid content and ground water condition, along with structural features like joints, fractures, etc., while resistivity profiling measures lateral changes in electrical properties of the formations. The 3-D resistivity approach provides more detailed spatial information of the geological setup for the design and operation of the core-drilling program.

Another option is seismic reflection or refraction, depending upon the need and required information. Now, with advances in seismic technology and their resolutions, seismic data may provide much useful information for the core-drilling program, operational parameters and optimum core recovery. So a seismic reflection method may be very useful in the core-drilling program, especially in sedimentary economic rocks and minerals like coal and uranium (strata bounded deposits), by inferring the stratigraphy of the region under investigation. The information that can be extracted from the analysis of seismic line attributes are depth and thickness of formations, lithological information (consolidated as well as unconsolidated), structural features and ground water. A surface geophysical survey is useful to decide, design and optimize a core-drilling program to get the required information.

The choice between seismic and resistivity method may be decided on cost-benefit analysis. These surveys are useful not only for the purpose of core-drilling designs, but also in further stages for the purpose of correlation of data between two boreholes for reserve estimation, mine planning, design and development in the mineral industry, and various geotechnical work like foundation evaluation and selection, tunnel and shaft site evaluation, and their design and development.

In deciding on a suitable core-drilling rig, factors that should be considered are the required depth of the drilling, final diameter of the borehole, and the results of previous case studies of used core-drilling rigs, depending on availability of the information. The number, diameter and length of the casing should be determined based on geological conditions to avoid caving, borehole collapse and problems in coring. The soft, weathered, loose and more heterogeneous formations to be drilled, the more numerous casing sizes required to sustain the borehole walls. Type, size, speed, bit pressure and water pressure should be adopted based on geological information to make the drilling operation economical and efficient. Choose whichever factors will provide the best core recovery, combined with good progress.

However, it has been observed that fast progress and good core recovery, as a rule, cannot be combined. The rotation speed, therefore, is set as high as the drilling machine and the drill rod string will permit without sacrificing stability, causing vibration, disrupting core recovery, wear and tear on tools, etc. So it is very important that the optimum set of drilling operational parameters, based on geological information as discussed above, are set, giving a good core recovery, good rate of penetration, and a moderate rate of wear.