The application of modified reaming technology in Mississippi Canyon, deep-water Gulf of Mexico (GOM) has had a beneficial impact on drilling costs. The ability to drill cement and associated downhole float equipment with steerable ream while drilling (DOSRWD) tools has become increasing important in deep-water GOM drilling. The extra trip required by conventional bi-center and eccentric tools adds costly "flat" time to a well's drilling program. Additional improvements in the eccentric bit's geometry and cutting structures has reduced the number of trips and significantly lowered drilling costs. Traditionally, when a ream while drilling system was used, the operator drilled the cement and float equipment with a pass through size bit, then tripped out of the hole to pick up the ream while drilling bottom hole assembly. For example, float equipment and cement in 9–5/8" 53.5# casing was drilled out with an 8–1/2" bit. A trip was then made to pick up a standard 9–7/8" SRWD bottom hole assembly. The DOSRWD's ability to drill cement and float equipment inside casing and then continue drilling ahead, plus the option to use either a tricone or PDC pilot bit depending on directional requirements and formation type make it the appropriate tool for the application. Water-based fluids are preferred in the upper hole section while synthetic mud is used deep in the wellbore. Because of balling concerns in WBM and stringent directional requirements, the operator utilizes metal bearing seal steel tooth pilot bits in most cases (IADC 117). The authors will document rig cost savings ranging from US$40,000 to $105,000 per trip in some applications with total savings exceeding US$388,000 in some instances. Introduction Mississippi Canyon (MC) Blocks 667, 711, and 755 are located 150 miles south of New Orleans, Louisiana in approximately 3000 ft of water. These blocks are considered deep water Gulf of Mexico and pose numerous drilling challenges to the operator.The well plan calls for a relatively large wellbore to be drilled through the reservoir sand to accommodate high flow-rate completions. To successfully penetrate overburden with high pore pressure, the operator has used several hole-opening systems in the area to drill an oversized hole in order to accommodate multiple casing strings in the upper well sections (Figure 1). If not for the narrow margin between pore pressure and fracture gradient in the area, the operator could have achieved borehole diameter requirements through the reservoir sand with a conventional casing design. To date, a total of six wells have been drilled in the aforementioned blocks. All but one of the wells has been directional in nature. Directional well profiles have been either build-hold or build-hold and drop (S shaped wells). The angle build portion of the wells usually takes place in the upper well sections (4000-ft to 5000 ft TVD). Two hole opening systems have been utilized to drill these upper hole sections:The borehole is drilled conventionally and then opened with an underreamer (i.e. drill a 14.75" hole to casing depth then open the hole with a 17" underreamer to set 13.375" casing.The hole is drilled with a 17" steerable ream while drilling system (SRWD). In MC Blocks 667, 711, and 755, formation above the producing horizon consist of gumbo, unconsolidated sand and sticky shales. The unconfined compressive strength of the gumbo and shales is usually below 1500 psi in the upper sections. Although the formation becomes harder at depth, the unconfined compressive strengths of both the shales and sands rarely exceeds 3500 psi (Figures 2 & 3).
The typical gas wells drilled in heavily explored Panola County of East Texas encounter the highly abrasive Travis Peak and Cotton Valley formations at around 6,300 ft. The Travis Peak is normally reached at 6.300 - 6.500 ft, followed by the Cotton Valley from 8,000-8,300 ft to TD. The Travis Peak and Cotton Valley are each about 1,700-2,000 ft in length. Until recently, the extremely abrasive sections in these formations severely limited the life of the IADC Class 537 through 737 roller cone bits used to drill these wells. Between severe gauge and cutting structure wear, the bits run previously were normally pulled after about 600 ft of drilling. The shortened life meant six to 10 bits were required to complete the two intervals. This paper describes the design and application of new hard formation rock bits that are drilling up to 50% more footage than the 627 and 737 types run previously. The new IADC Class 647 and 747 roller cone bits incorporate new insert geometries and quantities. along with improved carbide grades and diamond-enhanced gauge protection. Today, the Travis Peak and Cotton Valley are being drilled with four bits, averaging more than 850 ft per bit. Along with the new bits, a revised mud program consisting of low-solid polymers contributed to the improved performance realized in recent Panola County wells. The authors will review the design process of the new bits. as well as presenting case studies of typical Travis Peak and Cotton Valley wells in Panola County and elsewhere in East Texas. Introduction The East Texas field was discovered in 1930 with the Joiner-Daisey oil well, Drilling had focused primarily on the shallower and highly productive oil bearing formations. While gas wells have been producing from the Travis Peak for more than 50 years, and gas sands were discovered in the Cotton Valley in 1937, it was not until 1968 that the first producing Cotton Valley gas well came on stream in Panola County. The Natural Gas Policy Act of 1978 and the Federal Energy Regulatory Commission's (FERC) tight gas sand policy of 1980 spawned interest in the gas reserves of the Travis Peak and Cotton Valley. Today, the focus of East Texas exploration is the marginal gas reserves. Fig. 1 outlines the East Texas area where most of the gas drilling takes place. Stratagraphically, the lower Cretaceous Travis Peak comprises fine-grained to very fine-grained sandstones, muddy sandstones and sandy mudstones The sandstones are highly cemented and consist primarily of quartz with quartz cementation. Owing to the extremely cemented nature of Travis Peak sandstones, as confirmed in laboratory tests, the rock fails by fracturing the individual quartzitic sand particles, which accounts for the extremely abrasive nature of the formation. P. 595
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