Well design, casing challenges in Gulf of Mexico subsalt drilling

Specialized pipe design key to battling collapse forces

Dorse Walton, Phillips Petroleum, Bruce E. Urband
Tubular Corporation of America

Casing design for the Mahogany subsalt wells.[37609 bytes]
Ovality in the casing sets up a net bending force. [25479 bytes]
When casing static stresses are superimposed over bending stresses, the combined stress load is shown. The first fibers of the pipe surface ID to exceed yield strength plastically deform.[22735 bytes]
Precise control of casing OD/ID quenching and processing to gain control over the shape of the stress-strain curve.[25458 bytes]

In 1993, Amoco Production and Phillips Petroleum Company independently drilled two subsalt wildcats which proved to have major implications for exploration and production in the Gulf of Mexico. Mahogany operator Phillips and Mattaponi operator Amoco demonstrated that large oil and gas bearing strata do exist beneath large horizontal, allochthonous salt sheets.

These two prospects in 1993, demonstrated the accuracy of 3D seismic technology and have led to a major drilling boom in the Gulf of Mexico. Phillips spudded its Mahogany wildcat well in mid-May of 1993 in about 370 ft of water on Ship Shoal 349. The Diamond M Odeco's Endeavor semisubmersible, drilled the well which penetrated a salt sheet about 3,500 ft thick. These sheets of salt, which are considerably thicker than that encountered on land (200 to 300 ft thick), impart high collapse loads on the casing. The collapse forces, in conjunction with the normally pressured and geopressured environments, above and below the salts, present very challenging casing design considerations.

Subsalt geology

The Mahogany area salts sheets are encountered below 7,500 ft with an average thickness of 3,800 ft. An interval of disturbed sediments, created when the sheet moved into place eons ago, extends 1,000-1,500 ft below the salt. Formation temperature at the salt base is about 170 degrees F.

Rotary sidewall cores in the salt were taken in the various wellbores and analyzed for stresses and chemical composition. The salt is very dense and competent. Fracture gradient checks performed in the salt by pump-in leak-off tests, approached 0.988 psi/ft (19 ppg). The disturbed sediments would not support a column of 16.9 ppg (0.879 psi/ft gradient) mud in some wells.

The fracture pressure is only 0.5 ppg EMW higher than the pore pressure in those wells. The pore pressure gradient decreased as drilling progressed below the salt, until the sediments became more competent and less affected by the salt barrier trap. Then, it started to increase again. The fracture gradient would decrease in conjunction with pore pressure.

Temperature and overburden are primary factors related to the creep and hole closure in a salt body. The thickness of the shales and the salt itself compounds the overburden effect at the base of the salt. The lateral flow of the salt creates a shear interface 200-500 ft below the top and above the base of the salt sheet.

Salt creep and hole closure are relatively slow at Mahogany and not a major problem during the drilling phase. Fractures have been encountered in the salt body containing oil and gas deposits. This situation creates another potential source of movement or shifting. Salt movement becomes a very important factor during the life of a producing well and it is usually more economical, in time and dollars, to plan for a successful well initially.

Casing loads

Casing strings and well bores both can be affected by shifting salt layers. Their reaction to salt layer movement is typically of two basic types:

When a well bore shifts, the entire casing string is shifted laterally and bows out.
Localized loading by shearing forces generated by lateral movement of salt sheets at varying rates creates the most difficult condition to anticipate. The collapse loads can be extreme.

The salt sheet forms an impermeable barrier that traps fluid and pressure below it. The pore pressure in the formations below the salt support everything above, and productive formations below the salt are over-pressured. Often, the casing set through the salt must be able to contain high shut in pressures of a producing well. This string of casing may have two hole sizes drilled below it, thus casing wear became a factor in the burst and collapse design.

Casing design

Drilling and producing hydrocarbons from the geological target is the objective of the drilling engineer. The target depth, formation fluid, type of formations to be drilled through, bottom bole pressure, and temperature are all factors of consideration for a casing design.

Production from the primary pay zone at Mahogany was oil. The optimum tubing size for oil at the depth, pressure, and temperature of the reservoir was 3 1/2 in. to 4 1/2 in. The design of the casing string started at the bottom of the hole with the desired tubing size and then the casing was designed back to the surface. The design allowed specific bit sizes to pass through, yet the outside diameter of the casing was limited by the hole size drilled.

The close tolerance of the outside diameter of the inner string to the inside diameter of the outer string of casing required the use of integral connections. These connections had to meet the burst and collapse ratings of the pipe body and suspend the weight of casing.

The first wells were drilled from a semisubmersible drilling rig. The subsea wellheads limited the largest size casing to 36-in. OD. The Mahogany wells were jetted in with either 36-in. or 30-in. casing. The conductor casing was usually 20-in., set in a 24-27-in. drilled hole. An 18 3/4-in. wellhead seal assembly was installed on the 20-in. casing and the largest bit size that could pass through it was 17 1/2-in.

The hole could be enlarged below the casing, but the largest casing size readily available to run was 16 in. The 16-in. surface casing was drilled out with a 14 3/4-in. bit. This hole section was drilled into the top of the salt formation and a 13 5/8-in. intermediate casing string was set. A 12 1/4-in. hole was then drilled through the salt to competent rock below. A string of 9 7/8-in. production casing was run and set in this hole section. The geologic target was drilled through with an 8 1/2-in. bit and a 7 5/8-in. production liner was set at total depth.

The first string of critical design was the 16 in. The concern with the surface casing was the ability to withstand collapse, should lost returns occur while drilling above and into the top of the salt. The minimum inside diameter could be no less than 14 3/4 in. to accommodate the 13 5/8-in. intermediate casing. It should be noted that 13 3/8-in. casing is standard size, but larger diameter pipe was required to obtain the strength needed and also allow a 12 1/4-in. bit to pass through it. The 9 7/8-in. production casing also was chosen to provide the additional strength needed with adequate bit clearance (8 1/2 in.). The 7 5/8-in. production liner was specifically designed for collapse with no special clearance inside diameter. When a deeper exploratory target was drilled, a 7 3/4-in. production liner was run to allow a 6 1/2-in. bit to drill below it.

The 9 7/8-in. production casing must withstand the lateral stresses of the salt movement, the overburden pressure at the base of the salts, the internal pressure from the producing formations below, and the suspension of the full string until cemented.

In the event that the formations fracture, the annulus fluid will drop while running the casing. If the casing stays full of mud, this adds to the weight of the casing. The calculated overburden gradient at the base of the salt in the Mahogany wells was 0.96 psi/ft. The casing collapse is designed for 1.0 psi/ft with a 1.2 safety factor, for critical exploratory wells. The expected shut in pressure of these wells is 9,000 psi. The casing must be able to hold this pressure if a tubing leak occurs. The burst design factor is 1.25 and the tensile design factor is 1.4.

Ship Shoal 349 Well #1 was temporarily abandoned in September 1993. The well was re-entered in December 1996 to complete and put on production. An 8 1/2-in. bit and casing scraper, sized for 9 7/8-in. casing, were run to the top of the liner with no problems.

After drilling the first well, the drilling program was amended for subsequent wells to tie the liner back to above the top of the salt and cement it in place. This step was taken as a precautionary measure against collapse and casing displacement over the life of the well. The 9 7/8-in. Ql25HC casing was still required because a full column of cement had not been successfully circulated to the top of the salt yet. This has been attributed to weak formations below the salt breaking down during the cement job.

Casing collapse mechanism

Localized loading of the casing by the shifting salt sheets puts extreme shearing collapse forces on the outside surfaces of the casing. As external forces are statically applied to the casing, a uniform compressive load is induced through the entire wall thickness of the casing.

During the installation of the casing, even under normal conditions, a slight ovality is elastically induced into the casing, which results in net bending forces. As these bending forces, as well as those resulting from normal variations in the wellbore are induced, the OD fibers of the pipe are placed in tension, while the fibers near the ID are placed in compression.

The bending stresses are distributed through the entire wall thickness of the pipe. The static stresses are superimposed over the bending stresses, resulting in a combined stress distribution. As the bending and external pressures increase, the first fibers of the pipe to exceed the yield strength are on the ID surface. Once the first fibers on the ID surface fail (exceed the yield strength and plastically deform), it causes an overall weakening of the pipe body and the failure mechanism continues like a domino effect, until catastrophic collapse failure occurs. This failure mechanism can be exaggerated further by the reduction in plastic collapse resistance resulting from normal axial tension.

Because the ID fibers are the first to fail in the collapse mode, it is important to maintain the highest degree of metallurgical quality on the ID surface. When the collapse resistance of the pipe has been exceeded, it usually results in a complete flattening of the pipe, unless the failure occurs when there is some type of bore restriction.

Casing selection

Because of the severe collapse loads anticipated by Phillips in the sub-salt wells drilled, over 74,000 feet of 9 7/8-in. OD X 62.80 #/ft (0.625-in. wall) Q125 enhanced high collapse (HC) casing was used. This casing product was designed and produced utilizing specialized casing manufacturing techniques to provide an enhanced minimum collapse resistance of 13,000 psi (standard API Ql25 casing has a minimum collapse rating of 11,140 psi).

Collapse testing was done by Southwest Research Institute in San Antonio, utilizing 79-in. long (8:1 L/D ratio) test samples obtained from pipe after manufacturing. To date, 24 collapse tests have been performed on the casing used by Phillips in the 7 subsalt wells in the Gulf of Mexico. The average or mean collapse resistance of all 24 samples tested was 14,796 psi.

Casing manufacture

The collapse resistance of API casing is based on three equations which address the effect on collapse resistance in the elastic, plastic and yield regions during loading. The equations, which can be found in API Bulletin 5C3, were derived in 1976 from 2,777 collapse tests representing six manufacturers of API casing. The equations, over the years, have been found to be somewhat conservative, due to the significant improvement in casing product quality since the mid-1970's.

The use of continuous casting, and modernized steel billet piercing mills can assure consistent pipe dimensions and metallurgical uniformity, compared to that produced during the testing used to develop the API collapse equations. In addition to the improvements achieved by the seamless pipe mills to produce uniform green tubes or shells for heat treatment, significant improvements in technology have impacted the heat treating of the green tubes to achieve the strength, ductility, and collapse resistance needed for today's subsalt applications. The use of some of the manufacturing techniques described below can assure consistent improvements in collapse resistance.

Stress (load) vs strain curve:

The most important metallurgical attributes of high collapse casing is maintaining a consistently high proportional limit and a steep sloped stress vs. strain curve. The steepness as well as the highest degree of elasticity are measured during tensile testing and are important properties to help deter collapse failures.
These material attributes are especially important for the ID fibers of the pipe which will be subjected to the highest stresses and will fail first. As stated earlier, the first fibers of the pipe to fail are on the ID surface and the failure mechanism progresses like a domino effect. Catastrophic collapse failure occurs when the remaining pipe wall cannot sustain the increasing collapse stresses and the tensile (ultimate) strength is exceeded. When this occurs, a complete flattening of the pipe is traditionally observed, although permanent bore deformation and restriction can be observed prior to complete pipe flattening.
The material's proportional limit and the slope of the stress vs. strain curve is controlled by the chemical composition of the steel, heat treating, and finishing - processes employed by the pipe manufacturer. As pipe wall thicknesses are increased, there is a greater difficulty in achieving the proper metallurgical uniformity through the complete wall thickness of the pipe, especially near the ID fibers.
Dimensional control: As with all well-designed and produced products, the importance of maintaining control of the dimensions is crucial to the overall product performance. Maintaining dimensional tolerances on pipe affects many things, from interchangeabi1ity (pipe fitting in the casing hanger) to performance. It is well documented that maintaining good wall eccentricity and minimal ovality are important in maximizing the materials collapse resistance when the failure mode is in plastic collapse.
These dimensional characteristics become less significant when good through-wall heat treating is obtained. Simultaneous ID/OD heat treating quench systems are now able to consistently produce high elastic limits with near ideal sharp knee stress/strain curves on and near the ID surface. These modern quenching systems can produce the highest degree of metallurgical uniformity, from ID to OD, and also assure high fracture toughness (Charpy impact strength).
Cold working, residual stresses: Steel cold working, typical of that imparted by traditional cold rotary straightening of pipe can reduce the pipe's collapse resistance. Both the Baushinger Effect and the residual stresses induced from cold rotary straightening, together, act to deteriorate the sharp knee (ideal) characteristic of the stress vs. strain curve. The use of hot sizing and straightening is used to prevent this deterioration.

Pipe manufacturing

Heat treating:

The heat treating process begins with the pipe being heated to a suitable austenitizing temperature in gas-fired walking beam furnaces. For Q125 HC products, this temperature is about l,600 degrees F. The use of computer controlled walking beam furnaces have demonstrated their excellent ability to uniformly heat pipe and assure that material is well soaked through the complete wall thickness. Tempering is also performed in computer controlled, gas fired walking beam furnaces with temperatures in the range of 1,100-1,180 degrees F, depending on the specific chemistry of each heat of steel processed.
Simultaneous ID/OD quenching: The quenching of the tubular is crucial in obtaining the highest degree of martensitic transformation near the ID surface. In simple terms, this translates to improved collapse resistance. The ID lance sprays high pressure water on the ID surface at the same time that high pressure water is being sprayed on the OD surface by a series of quench ring assemblies.
The ID lance design assures that the ID fibers have a high proportional limit (yield strength). The ability of the steel to he cooled from the ID and OD simultaneously enables the greatest amount of heat removal and results in material which exhibits a near ideal (sharp knee) stress vs. strain curve.
Traditional OD only quench systems may not provide the quench severity necessary to assure optimal transformation on the ID surface where it is so crucial for improving the collapse as well as the burst resistance. This is further exaggerated by the industry's need for thicker wall casing to sustain higher burst and collapse loads. Besides measuring a pipe's collapse resistance and yield strength characteristics, through-wall hardness tests and toughness also provide excellent quantitative measurements of the effectiveness of the quench system for the material being heat treated.
Hot sizing/straightening: As discussed previously, cold working will deteriorate the sharp knee, high proportional limit characteristic of well austenitized and quenched product. To reduce the detrimental effects of cold working the pipe, enhanced collapse products were sized and straightened in the hot condition after tempering. The deterioration of the stress vs strain curve is a function of the temperature of the pipe at the time of straightening, the amount of plastic deformation needed to straighten the tubular, and the specific direction of the stresses imparted into the tube during the straightening process. Quality control procedures were used during the production of the high collapse pipe used by Phillips to assure that the Ql25HC casing exited the sizing and straightening equipment above 730 degrees F.

Properties

Chemical properties:

The high collapse Q125 casing was produced by the continuous casting practice utilizing a clean steel ladle metallurgy practice. The green tubes had a chemistry which conforms with the AISI 4130 modified analysis and meets the API Q125-Type 1 criteria. Sulfur contents were .010% or less, with the typical product sulfur around .005%. A typical percent composition by weight product analysis was: carbon (.25), manganese (.55), phosphorus (.010), sulfur (.006), silicon (.25), chromium (1.00), molybdenum (.30), nickel (.01), and vanadium (.04).
The high collapse casing used by Phillips was subjected to the standard tests required by API Specification 5CT. The material met the standard API-Q125 requirements as well as an enhanced minimum collapse resistance of 13,000 psi.
Tensile Properties: Eighty full-wall thickness strap tensile specimens were tested in accordance with API Spec 5CT requirements for Q125-Type 1 casing. A statistical analysis of the yield strengths per ANSI/ASQC Z1.9-1993 double specification limit method was performed.
The stress vs strain curves for the Ql25HC material provided Phillips had a near ideal shape, indicating that the material was thoroughly quenched to obtain martensite. As explained earlier, the characteristics of the stress vs strain curve have a profound affect on collapse performance. A statistical analysis of the tensile or ultimate strengths was performed per ANSI/ASQC Zl.9-l993 single specification limit method.
Fracture toughness properties: Fracture toughness is a measure of the material's ductility or ability to arrest crack propagation. Charpy impact tests are used to measure the amount of energy necessary to break a notched bar specimen. Two hundred forty full size, transverse Charpy V Notch (Cv) impact tests were performed on the QI25HC used by Phillips. All specimens were tested in accordance with API Spec 5CT requirements at 320 degrees F and exhibited a 100% ductile shear fracture appearance when examined visually. A statistical analysis of the Charpy impact strengths of the measure of the material's fracture toughness was performed per ANSI/ASQC Zl.9-l993 single specification limit method.

Collapse testing

The collapse testing of the steel casing used to develop the API collapse equations were performed in relatively short pressure vessels as compared to that available today. Although it was generally recognized at the time that collapse testing results are affected by short collapse test samples (less than an 8:1 L/D ratio), the lack of longer pressure vessels at the time precluded testing longer length collapse specimens.

The collapse vessels used were about 30 in. long. A 2:1 L/D ratio test specimen standard was established to allow some standardization for collapse specimen length to accommodate the casing sizes up to 13 3/8-in. OD.

Although there are still many manufacturers around the world performing collapse tests with the less conservative short (19-in. long 2:1 L/D ratio) specimens, the casing provided to Phillips was tested using a 79-in. long collapse test specimen. This equates to an 8:1 L/D ratio.

When comparing the collapse resistance of the same joint of casing using a long collapse test specimen (8:1 L/D) versus a short collapse specimen (2:1 L/D), the shorter specimen collapsed at a higher collapse pressure. The use of short test specimens may inappropriately give the drilling engineer a false sense of security because the casing string acts as an infinite member when evaluating casing string forces, not 19 3/4 in. long. In critical applications, especially for Phillips, where collapse loads are substantial, the use of longer collapse test specimens was determined to better represent the real loads to be sustained.

Testing

API Spec 5CT requires that all Ql25 casing be hardness tested through the entire wall thickness to demonstrate the uniformity in hardness through wall. A one quadrant 9-point hardness traverse was obtained on each sample, which was hardness tested. Over 720 hardness readings were obtained on the casing provided Phillips. The hardness readings on all test samples were less than the permissible API specification variance of three Rockwell C.

Forty six collapse specimens (8:1 L/D ratio = 79 in. long) were tested by Southwest Research Institute to assure unbiased test results for the casing provided to Phillips in the Gulf of Mexico. All tests were conducted at room temperature using standardized SWRI/TCA sample preparation and testing procedures to assure accurate and consistent testing. These test procedures were developed based on the API research projects on high collapse casing performed in the 1980s at Southwest Research Institute.

All test specimens were secured in fixtures that are designed and fabricated so that no hydrostatic end loads are transmitted to the test specimen during collapse testing. This is done to assure that the ends of the specimen are not restrained which can affect the test results.

The utilization of Q125HC casing with enhanced collapse resistance has provided Phillips Petroleum an additional tool to help prevent casing failures in the existing seven wells drilled in the subsalt. Manufacturers who use specialized manufacturing techniques like simultaneous ID/OD quenching and hot sizing and straightening, as well as tight manufacturing and quality control requirements, can produce casing with collapse resistances greater than that predicted by the API equations in API Bulletin 5C3.

Editor's Note: This paper was presented at the Subsalt '97 conference in Houston. A list of references and additional graphics, not presented here for spatial reasons, can be obtained from the authors.