OIL FIELD SLIM HOLE DRILLING TECHNOLOGY IMPROVING

Nov. 23, 1992
Recent advances in slim hole drilling technology have improved the application of this drilling technique to oil and gas exploration and development wells. These advances, which were described at the Society of Petroleum Engineers' annual meeting in Washington D.C., Oct. 4 7, include: Optimization of slim hole drilling hydraulics Application of a small particle weighting agent to improve well control and coring operations Use of slim hole techniques to drill horizontal wells Use of a new

Recent advances in slim hole drilling technology have improved the application of this drilling technique to oil and gas exploration and development wells. These advances, which were described at the Society of Petroleum Engineers' annual meeting in Washington D.C., Oct. 4 7, include:

  • Optimization of slim hole drilling hydraulics

  • Application of a small particle weighting agent to improve well control and coring operations

  • Use of slim hole techniques to drill horizontal wells

  • Use of a new polycrystalline diamond compact cutter to allow economical re entry of small diameter wells in hard rock.

Slim hole continuous coring and drilling is becoming more accepted as a viable drilling method, especially as exploration budgets become smaller. Typical applications for slim hole equipment include drilling in frontier areas where logistics can be a problem and re entry operations in which the existing well has a small diameter.

Typically, slim hole drilling operations use technology borrowed from the mining industry. The rigs are smaller and drill with much higher rotational speeds.

Definitions of slim holes vary from a well with 90% drilled with a diameter of less than 7 in. to a well with 70% drilled with less than 5 in. A goal of slim hole, however it is defined, is the drilling of a well with a diameter smaller than that used on conventional wells in the area. The reduced diameter helps cut rig time and cost and reduces the cost of the tubulars.

Another goal of slim hole drilling is the ability to retrieve cores from the entire well during drilling.

SLIM HOLE HYDRAULICS

A model based on fluid mechanics helps optimize slim hole drilling hydraulics by taking into account mud rheology, drillstring rotation, and the eccentric position of the drillstring.

Drilling hydraulics can become a problem in sedimentary formations drilled with slimhole drilling equipment because of the high rotational speeds and small annuluses, according to R.A. Delwiche of DB Stratabit, consultants M.W.D. Lejeune and P.F.B.N. Mawet, and R. Vighetto of Total in the paper: "Slimhole drilling hydraulics."

The crucial areas include the lifting of cuttings in the annulus, well bore stability bit cleaning, differential mud pressure in the annulus, and the hydrodynamic lubrication between the drill rods and the well bore.

The annular mud velocity profile must be kept as uniform as possible to ensure that cuttings are transported to the surface without balling up in the annulus. In general, a mud velocity of about 0.5 m/sec (1.6 fps) should keep cuttings moving faster than the sedimentation velocity in the annulus, according to the authors.

In soft sedimentary formations, a laminar flow regime is important in the annulus to minimize the shear stress close to the borehole wall. High shear stresses close to the wall can cause erosion and caving, reducing the well bore support of the drill rods.

In slim hole drilling, the trajectory of the mud and cuttings becomes helicoidal, and not straight as in conventional wells. The rotating rods force the mud to rotate because of a viscous effect, the Couette effect.

Another slim hole drilling hydraulics phenomenon, the crescent effect, occurs when the rods rotate eccentrically in the well, with the eccentric geometry influencing the pressure losses in the annulus. Although eccentricity occurs in conventional wells, the effect on pressure losses is not as great.

Thus, the dynamic component of the annular pressure is much more important in slim hole drilling than in conventional drilling.

WEIGHTING AGENT

A finely ground material has found a specific application for weighting drilling fluids used in slim hole operations. In slim hole continuous coring and drilling operations, conventional weighting materials may become deposited on the inner wall of the drillstring because of the high rotational speeds (200 1,000 rpm). These deposits on the inner wall of the drillstring cause problems with retrieving cores by wire line.

A pigment grade barite was used to weight cationic brine muds in excess of 14 ppg with acceptable viscosity. Typical oil field barite ground to an ultrafine size should work comparably to the pigment grade barite. The pigment grade is purer than conventional oil field barite, and it is predominantly used as a paint pigment or plastics filler.

The pigment grade barite does not interfere with wire line core retrieval within reasonable operating ranges of drilling parameters, according to S.B. Randolph, S.H. Walker, G.A. Young, and D.S. Dorrough of Amoco Production Co. in "Use of a unique weighting agent for slimhole drilling."

In slim hole wells, well control can become a problem because of the small annular clearances, often less than 1/2 in. Well control is typically maintained through either dynamic well control methods or heavy fluids. The weighted brines are expensive, and conventional coarse barite can form deposits or bridges in the annulus.

The pigment grade barite can be obtained through certain suppliers of industrial solids. Although it costs somewhat more than conventional barite, small amounts are used, and little mud is lost in slim hole wells.

The pigment grade barite has a size distribution of 80% less than 1 m and 50% less than 0.18 m. Conventional coarse barite has a size distribution between 80 m and 1 m.

The pigment grade barite can formulate muds with low viscosity, which is acceptable in slim hole drilling because the cuttings are fine and easily transported up the annulus. This barite does not readily settle out of thin drilling muds because the particles are so small.

The weighted mud can be pumped through a centrifuge rotating at reduced speed to discard the low gravity solids without loss of too much of the fine grind barite. The pigment grade barite was mixed into the mud system with a hopper and standard mixing procedures.

HORIZONTAL WELLS

Slim hole drilling technology can reduce drilling costs in horizontal wells because of the use of smaller drilling rigs or workover rigs, smaller casing sizes, and minimized drilling wastes.

However, the cost savings achieved from slim hole drilling can be offset by increased mechanical failures, reduced lateral hole length, and lack of directional control.

The decision to drill a slim hole (3 7/8 4 3/4 in. hole compared to a conventional hole of 6 8 1/2 in.) requires involvement from all departments associated with the well to prevent the limitations of the smaller well bore from exceeding the benefits, according to C.R. Hall and A.B. Ramos Jr. of Oryx Energy Co. in "Development and evaluation of slimhole technology as a method of reducing drilling costs for horizontal wells."

The limitations of downhole equipment have often precluded drilling lateral intervals with less than 6 in. diameter. Until recently, the cost savings of smaller tubulars would be more than offset by lower rates of penetration or directional control problems.

In 1991, Oryx drilled a number of re entry horizontal slim hole wells, two of which were drilled with a coiled tubing rig. The conventional rig operations used a wet connect steering tool system and a modified power swivel assembly through which the steering tool cable could be run. The results from these wells were promising.

Oryx then drilled two horizontal slim holes in the Pearsall field in south Texas. The program used a small drilling rig for drilling from surface to the intermediate casing point. This rig was released and replaced with a workover rig. The small drilling rig could drill the upper hole more quickly than the workover rig, and the less expensive workover rig could handle the small tubulars for the horizontal section more easily.

Recently developed small diameter (3 3/8 in.) measurement while drilling (MWD) tools were used instead of a wet connect steering system.

The first well (4 1/2 in.) encountered a high number of mud motor failures, some minor problems with the small MWD tool, and difficulties with fishing because of the small hole and high angle (20/100 ft).

The second well (4 3/4 in.) used larger mud motors which could better handle the torque of the polycrystalline diamond bits. This well also used 2 7/8 in. drill pipe in part of the curve to increase the weight on bit. With more torque available and more aggressive bit designs, the rate of penetration on this well increased 125% more than that for the first well.

The costs for this slim horizontal well were 20% less than the costs for the first well and 32% less than the costs of a conventional design.

DOME PDC BITS

Dome polycrystalline diamond compact (PDC) technology on small diameter bits and underreamers has opened opportunities for re entering old producing wells for deepening and recompletion in the Permian basin.

The dome PDC has a radius of curvature across the diamond table rather than the typical flat table. This design reduces torque and dissipates heat better than conventional PDCs. Because the structure is very impact resistant, the dome PDC has potential for harder rock applications, according to J.A. Carter and M.E. Akins of Chevron U.S.A. Inc. in "Dome PDC technology enhances slim hole drilling and underreaming in the Permian basin."

The dome PDC cutters were used in slim hole operations (3 1/4 4 3/4 in. diameter) wells in the Grayburg/San Andres formation in Lea County, N.M. The 3/8 in. cutters were used on bits and underreamers in dolomitic formations where conventional PDC cutters have failed in the past.

The dome PDC bits have become an economical tool for the deepening of old wells with 4 in. or 5 in. liners at depths ranging from 3,500 to 4,500 ft in the Permian basin.

The weakness in conventional small diameter roller cone bits has been the bearings, not the cutting structure. The bit runs have been short, often with junk left in the hole. Conventional flat PDC cutters have also been tried, but these were destroyed early in the drilling, resulting in a high cost per foot.

A number of the older wells in the Permian basin have been unitized in water floods. Some of these wells, completed in the 1930s and 1940s, require cleaning out and deepening before becoming part of the waterfloods. A typical well bore configuration is a 5 1/2 in. liner with an open hole completion or a 4 in. liner.

Underreaming prior to running the liner has been effective in improving cement bond and offers the opportunity to run larger diameter liners. This task is difficult with conventional slim hole roller cone technology because the rigorous drilling conditions of the hard rock formations wear the tools fast, often leaving junk in the hole.

The use of dome PDC cutters on underreamers in these wells has reduced costs by more than 60% compared to similar costs for roller cone tools. The savings resulted mainly from an increase in penetration rates to more than 30 ft/hr from 10 ft/hr.

Additionally, the dome PDC tools lasted longer in the well.

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