SLIM HOLE DRILLING-1 MINING TECHNIQUE FINDS APPLICATIONS IN OIL EXPLORATION

E. P. Deliac, J. P. Messines
Elf Aquitaine
Paris

B. A. Thierree
Exploservices
Paris

Oil exploration companies have taken increased interest in slim hole drilling as practiced by the mining drilling industry in small diameter coring.

In addition to providing cores for the entire well, the core hole drilling technique is an attractive exploration method because a well can typically be drilled for approximately 30% less than the cost of a conventionally drilled well. These savings increase even further for remote locations because the smaller core hole rig package reduces logistics and transportation expenses.

The worldwide status of slim hole drilling and its applications to the oil industry were surveyed by Exploservices. Based on this survey of slim hole drilling, part of which is documented here, a major research and development project was recently launched by Elf Aquitaine. This multidisciplinary project joined explorationists with drilling and reservoir engineers to analyze existing slim hole techniques and improve them for oil field applications.

SLIM HOLES

The most commonly accepted criterion for slim holes is a well diameter smaller than 8 1/2 in. According to one definition, the criterion becomes "one in which 90% or more of the length of the well is drilled with bits less than 7 in. in diameter."1 This definition seems rather restrictive because it eliminates most deep slim hole programs.

Thus, the definition of slim hole wells should be widened by introducing relationships between depth and well bore diameter. In general terms, an oil industry well is then considered as slim hole only when its diameter is drilled smaller than what it is typically drilled in a conventional oil well at the same depth. Consideration should also be given to the clearances between the well bore and string that are less than what is usually found in oil wells.

For instance, an 800 m (2,624 ft) well drilled with a 12 1/4-in. bit and 5-in. drill pipe is considered normal for the oil industry. However, the same well becomes "slim" at that depth while using a 101.3-mm (4 in.) core bit and 94-mm (3.7 in.) drill rods.

Slim hole drilling is standard practice worldwide in the mining industry. Destructive drilling (i.e., reducing the rock into cuttings) is typically used for shallow formations at depths less than 500 m (1,640 ft). For the remainder of the well, most slim hole drilling produces continuously retrieved cores.

An exploration well should be understood as both the hole itself and the material or raw data which is later processed to yield information about the fluids and the formations. It is here that slim hole exploration wells (SEW) become valuable by providing extensive tangible data. Slim hole, continuous cable coring rigs (SHCR) are particularly well-adapted tools for drilling and developing such wells.

HISTORICAL REVIEW

The mining industry has used continuous cable coring for many years because this is the most effective investigation method that gives precise indication about the ore contents in formations. The technology has been developed independently of the petroleum industry, using well-adapted and often original solutions. The performance of the mining rigs is quite remarkable considering the depths reached-5,423 m (17,788 ft) in South Africa-as well as the penetration rates and costs.

Oil companies made their first attempts at using mining technology for oil exploration in the early 1970s. In Canada, Gulf Oil Canada cored 3,530 m (11,581 ft) in 180 days and obtained a final well bore diameter of 76 mm (3 in.) with a Heath & Sherwood HS150 slim hole, continuous cable coring rig.2

In 1979, three Australian oil companies (Western Mining, Poseidon Oil, and Australian Hydrocarbons) used slim hole, continuous coring rigs combined with seismic operations to explore a vast, remote area that previously contained only four wells in an area over 32,000 sq M.3 They undertook a campaign that drilled 31 stratigraphic wells in 1981-84 with a maximum depth reached of 1,857 m (6,092 ft). Several reservoirs, mainly in carbonate formations, were discovered. This can probably be considered as the first successful application of the SEW concept.

In the 1980s, Gulf and Conoco Inc. used SHCRs for heliported oil exploration operations in Papua New Guinea.4 These rigs made discoveries at depths of 1,100-2,700 m (3,608-8,858 ft) in 76-mm (3 in.) diameter well bores. Also in the 1980s, Gulf and Chevron Corp. started programs in Texas and New Mexico reaching a depth of 2,980 m (9,776 ft) with a 76-mm (3 in.) diameter well bore.5

Later, Amoco Corp. initiated an extensive research and development program to study the possibilities of continuous coring using the SEW concept.1 Several wells were cored with a Longyear HD600 rig, reaching a maximum depth of 2,930 m (9,612 ft). Amoco systematically analyzed all facets of slim hole drilling technology (bits, mud, etc.) and developed the aspects which were neglected by the mining industry.

In particular, Amoco created a model for hydraulics, developed a system for kick detection, and wrote procedures for proper kick control. It reviewed the classical exploration approach of seismology and conventional drilling and replaced it with a new combination of seismology and continuous coring drilling. It also developed the complete laboratory equipment needed for immediate processing of cores.

Recently, Texaco Inc. used a Longyear PM603 SHCR for oil exploration in Paraguay, reaching about 3,000 m (9,144 ft) with a 76-mm (3 in.) well bore, and subsequently in Tanzania, reaching 1,940 m (6,368 ft). This will be covered in the second article of the series.

RIG DESIGNS

Conventional oil industry drilling rigs are quite effective in destructive drilling, even in slim holes. However, these rigs can only core limited lengths with limited efficiency because it is typically necessary to make a round trip to recover each core barrel.

Conversely, mining drilling rigs drill destructively only in the surface formations, always drill slim hole diameters, and core continuously for most of the well. Mining rigs and their equipment are quite effective for these operations because:

• The smaller size reduces location area (about 9,000 sq ft with mud pits) and makes them much lighter than oil rigs.

• The reduction in bulk allows helicopter transport or trailer mounting.

• The power required is low, typically 300-400 hp.

• The drillstring handling system uses a chuck clamping head to provide rotation and bit weight.

• The all-hydraulic design allows flexible commands of weight-on-bit (WOB), drill rate, and rotary speed. The automated hydraulic controls are necessary to ensure protection of the fast-rotating drill string, especially in case of a sudden variation of torque.

• The continuous coring system allows recovery of the inner tube of the core barrel by using a wire line and without tripping the drillstring. For instance, at 2,000 m (6,561 ft), drilling is only interrupted 15-20 min for every 18-ft core recovered.

• The high rotation speeds, commonly 200-500 rpm and up to 2,000 rpm, are specially suited to the hard metamorphic rocks encountered in the mining industry.

• The coring bits are impregnated-diamond or surface-set-diamond drill bits, specially designed for high rpm and low WOB for drilling hard formations.

CONTINUOUS CORING RIGS

The main available rigs can be classified in three groups:

1. Slim hole destructive-drilling rigs. Many of the small oil drilling rigs or workover rigs, such as those made by Failing, Helmerich & Payne, Parker, and others, can drill slim holes, but these rigs are not necessarily designed for continuous coring.

   Additionally, one must note the original approach of the Microdrill rig which was conceived merely as an oil rig with reduced dimensions. This category excludes small mining rigs that are limited in depth, such as those made by Wirth or Saltzgitter.

2. Continuous cable coring mining rigs. These rigs, for the most part, have no rotary table and are designed for coring most of the well length. Several companies supply rigs able to reach depths of 2,000-6,000 m (6,096-19,685 ft): Longyear, Universal Drill Rigs (JKS Boyles), Bournedrill, Universal HS, Boart, etc.

3. "Mixed" oil drilling rigs. During the last few years, several attempts have been made to transform oil rigs into coring rigs by lifting them with mining rods, wire line core barrels, and chuck clamping systems. Parker Drilling has completed the hybridization of four oil rigs which are now capable of reaching depths around 6,000 m by core drilling. For example, Forasol transformed a Walker-Neer rig, named Foramatic, by introducing an extensive use of automation similar to that on SHCRS. Additionally, Cofor has modified a Massarenti MR700.0 oil rig for continuous coring operations.

DRILLING OPERATIONS

The technological approach of the mining industry for drilling and coring slim holes is considerably different from the typical oil industry approach. Because of the differences in equipment and its applications, an operator using mining equipment (rigs or rods) will have to adapt its techniques in relation to its knowledge of the technology and to the specifications required for safety, especially in the area of kick detection and control.

The reduced drill rod/well bore annular clearance and the fast rotation of the drillstring require modification of the usual concepts regarding kicks. Because the annulus contains such a small fluid volume, a kick poses a serious threat of emptying the well. Thus, kicks must be detected early, after only a small influx of fluid. The standard procedures for controlling a kick by circulating the influx after checking surface pressures do not apply easily. The pressure drop distribution is inverted in comparison to that found in conventional oil wells. Indeed, as a result of the small annular clearance, most of the pressure drop occurs in the annulus section. In contrast, the pressure drop occurs inside the pipe in those wells drilled with conventional drilling rigs.

MUD HYDRAULICS

Reduced annular clearances and fast pipe rotation contribute to turbulent flow over most of the well. Thus, many classical well hydraulic models become inadequate.

The solids content in the mud must be kept as low as possible to avoid forming sludge inside the rods. Sludge built up on the rod inner wall makes core barrel recovery difficult and sometimes impossible. Therefore, solids control equipment, such as centrifuges, should be systematically implemented.

DRILL RODS

The thin-wall coring rods have square-cut threads and flush joints with annular clearances less than 1 cm (0.4 in.). The smaller diameters make it possible to use the rods as retrievable intermediate casing in the open hole. Fig. 1 is a schematic of a typical slim hole with rod strings run as casing.

The mining industry uses a flushed rods system that does not comply with oil industry specifications. Standard rod sizes (A, B, N, H, P, and S) are listed in Table 1. Mining standards are substantially lower than American Petroleum Institute (API) standards, particularly in terms of makeup torque and mechanical strength of rods and couplings. In some circumstances, the limits of mechanical strength may force the oil operator to stop operations early.

CEMENTING

Cementing volumes can be reduced using drill rods as a retrievable casing string. Because of the high pump pressures required to overcome the increased friction in the small annulus, cementing operations might become difficult with respect to channeling behind pipe and fracturing of weak formations. Reaming the well before running casing may offer one solution to this problem. The small volumes of cement required make it possible to search for more adaptable substitutes for working in those small diameters.

CARE PROCESSING

One of the main intentions of continuous coring operations is the gathering of as much information as possible. Thus, immediate core processing and analysis must be particularly well organized.

With a proper core lab setup, a complete set of data could be measured on site, including such parameters as: porosity, permeability, saturations, geochemistry, gamma ray, magnetic susceptibility, fluorescence, and hydrocarbon analysis. Unfortunately, a suitable well site service for core processing does not seem to be available yet.

Moreover, mud logging operations are more productive because cores are analyzed instead of cuttings, which are virtually nonexistent when coring.

Core samples are larger and less contaminated by mud with a precise depth of origin known.

On site core processing should make it possible to reduce to a minimum the amount of logs required. Most of the standard electric line logs are available for diameters down to 2 7/8 in., and production testing equipment is available down to 3 in. However, the compatibility should be checked with the processing methods used.

CHALLENGES

The adaptation of slim exploration wells to the oil and gas industry brings new challenges to the oil field:

• Technology must improve some of the problems and limitations of slim hole drilling and improve the real-time analysis of cores and logs.

  Furthermore, formation testing in small-diameter wells needs to be considered and studied.

• Safety of the rig and crew presents additional problems in areas including kick control, early gas detection, and implementation of explosion-proof standards for mining rigs.

• With the development of core hole drilling as a tool for the explorationist and reservoir engineer, exploration strategies may change as a result of the large amount of raw information immediately available at reduced costs. This approach may result in a redistribution of the respective input from and expenses to the geophysical and drilling departments.

OPTIONS

SEWs should be analyzed as an option for an exploration project in the following circumstances:

• If a significant part of the drilling budget is for logistics

• If difficulties are expected as a result of climate, environment, or site access

• If seismic operations become complicated, costly, or difficult to interpret

• If a well is reentered for deepening

• If there is a need for particular or precise information supplied by continuous cores, including: stratigraphy, sonic velocities, formation age and maturation, source rock and cap rock nature, reservoirs, and formation fluids

• If a field is evaluated for extension (for example, before expiration of permits).

Because of the present limitations of slim hole exploration, especially concerning safety, kick detection, and well control, it appears that SEWS are better adapted for geologic appraisal purposes than for wildcat exploration.

THE FUTURE

Elf Aquitaine acknowledges that a major breakthrough in drilling could result from the successful development of SEW technology.

To develop a fully operational SEW technology, Elf Aquitaine has planned industrial experimentation of slim wells.

The preliminary trials of slim wells in France and Africa already show promise, confirming that SEW technology can provide more wells for a given budget with each well yielding more information.

REFERENCES

1. Walker, S.H., and Millheim, K.K., "An innovative approach to exploration and exploitation drilling: The slim hole high speed drilling system," Journal of Petroleum Technology, September 1990.

2. "Small-diameter exploratory holes may gain more attention," OGJ, Feb. 22, 1971, pp. 86-87.

3. Ashton, S.M., "Slim hole drilling in the Canning basin: philosophy and application," Geological Society of Australia and Petroleum Exploration Society of Australia Ltd., 1984.

4. Macfayden, M.E., Johnston, K.A., and Boiyngton, W.H., "Slim hole exploration drilling program in Irian Jaya, Indonesia," IADC/SPE 14733, February 1986.

5. Rhodes, E., Svendsen, W., Polk, G., and Branson, T., "Drilling rig completes continuously cored well in Delaware basin," AAPG Explorer, July 1986.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.