Insufficient borehole pressure integrity (BHPI) is a significant drilling challenge in deep, high-temperature, high-pressure (HTHP) wells in south Texas, as it is in many wells. Shales and/or sands weakened by depletion, leaking faults, or unfavorable rock properties result in lost returns when mud weights are close to pore pressures. In one field, short (~50 ft) transitions from normal (11 lb/gal) to overpressured (17.5 to 18.0 lb/gal) Frio formations compound the severity of this challenge. Setting casing to isolate normal-pressure from high-pressure zones can be problematic if faults exist at the casing shoe and/or the cement job does not provide a good hydraulic seal. In one case, the intermediate casing shoe failed to test, and conventional cement squeezes were unable to correct the problem. In the productive portion of the well, preventing skin and or formation damage in an interval that had a wide range of pore pressures (8.5 to 17.8 lbm/gal), was a major concern with any treatment option to increase borehole integrity. This paper describes successful applications of new BHPI treatment materials and methods for increasing borehole integrity. BHPI treatments have allowed higher drilling and cementing circulation rates. This has helped optimize drilling performance and improve well conditions during cementing operations, which has resulted in improved primary cementing success. It has been suggested that skin damage in the zones of interest can be minimized since BHPI treatments can be designed and targeted to only enter areas with low BHPI. In one case, a BHPI treatment entered a low-pressure productive interval, which, after a planned stimulation program, did not seem to affect production performance. In another case, after BHPI treatments helped increase wellbore integrity, the productive interval in one well was successfully cemented without requiring a drilling liner, which would have limited completion flexibility. A theoretical rock mechanics model is discussed to help explain how the new BHPI treatments can rapidly and substantially increases the pressure integrity of holes located across both sand and shale formations. Minor BHPI filtrate invasions during tests in high- and low-permeability sandstone cores should explain why the new BHPI system also limits formation damage. Introduction Many types of formations can have poor BHPI integrity immediately below the casing shoe and deeper in the hole to the next casing-seat depth. This lack of pressure sealing, structural integrity to contain planned bore-hole pressures may be the result of natural in-situ stresses that cause weak BHPI points or defects in rock such as natural fractures and leaking faults. Drilling induced stresses that create new fractures or open sealed faults make up the balance of causes for low BHPI along with a significant number of chemically sensitive formations that weaken upon exposure to drilling fluids. Equivalent-circulating-pressure (ECD) and swab/surge pressures during drilling, tripping drill pipe, running casing, and cementing may exceed these low BHPI values. In the drilling cases, problematic conditions can occur, such as severe lost circulation, inadequate hole cleaning, lowered fluid column pressures, and subsequent formation fluid influx. During drilling, problematic conditions can occur, such as severe lost circulation, inadequate hole cleaning, lowered fluid-column pressures, and subsequent formation fluid influx. Exceeding BHPI values during primary cementing can jeopardize zonal isolation and casing support. These incidents often increase well development costs by forcing operators to set casing early, run a drilling liner, use a contingency casing string, and perform remedial cementing. In some wells with known low BHPI conditions, such as deepwater and HTHP wells, budgets must account for additional pipe strings necessary for drilling and completing the well. In addition, a significant number of well control problems occur from lack of BHPI.
Wells drilled into the deep Bossier formations of the east Texas Hilltop Field encounter low-permeability, gas-bearing formations at over 15,000-psi pressure and 400°F temperatures. The wells require high-pressure fracture stimulations and extreme production drawdown to produce at economic rates. Wellbore temperature variations occurring between stimulation and production operations are extreme. The gases in these formations are also highly corrosive. Two of the first three wells completed in this area failed from casing collapse during completion operations or within the first few weeks of production. Finite element analysis (FEA) modeling coupled with log-derived formation properties confirmed that the extreme stresses applied to these wells rendered previous casings and cement sheaths "under-designed." Using an approach that combined formation, casing, and cement mechanical properties into a system, the wells were redesigned. Detailed thermal and mechanical modeling of all wellbore operations resulted in redesigned casings and a cement sheath more applicable to the extreme loads being exerted. Minor changes were also implemented to the job placement procedures to lessen the loads placed on the cement sheath. High-strength, corrosion-resistant casings and specialty cement designs were successfully used on the first two wells. Since those wells have been on production, additional wells have been drilled and completed using incrementally-simplified designs. All the wells have withstood multiple stimulations at treating pressures exceeding 14,000 psi, production test drawdowns at the perforations of over 13,000 psi, and temperature changes estimated at more than 300ºF. The wells have withstood these extreme pressure and temperature changes without failure of either the casing or cement sheath. The cement and casing designs employed have proven competent for the high-pressure, high-temperature (HPHT) conditions encountered. The successful design methodology couples well-specific casing and cement designs into a system capable of surviving the extreme pressure and temperature conditions imparted on the well during stimulation and production operations of deep, low-permeability HPHT gas sands. Introduction Construction of deep gas wells involves a large capital expenditure, and they are typically prolific wells. In addition, remedial work can be very costly, not only in terms of lost production, but also in the cost of materials and services needed to perform the work. Catastrophic well failure, although rare, does occur and can doom remaining reserves in place when it happens. Hence, there is a large incentive to do things right the first time. The traditional focus of the cementing job of designing adequate slurry properties and getting the slurry properly placed still applies, but that is only the beginning. As these wells are completed and produced, the cement sheath is designed to survive extreme stresses. Wellbore longevity will depend not only on how the cement sheath is designed to impart maximum sealing properties, but also on how it behaves when coupled to the casing and formation during all well operations. All operations and their associated timing with respect to the completeness of the cement hydration are "fair game" for investigation, including:Continued drilling operations (in the case of intermediate casings).Completion operations (e.g. completion fluid circulations and stimulation treatments).Well testing (e.g. pressure testing, severe drawdown tests, etc.).Access to various annuli for pressure control during thermal changes.The effects of gradual drawdown during long-term production.
The objective of this research was to investigate the potential impact of expandable casing technology on the remediation of Sustained Casing Pressure (SCP). Varying magnitudes of SCP exist in the Gulf of Mexico (GOM), where over 80% of casing strings exhibiting SCP are production and surface casings, representing a great technical, economic, and environmental risk. Situations in which SCP is observed usually result in costly and frequently unsuccessful remediation efforts. The technique proposed in this project with expandable casing can be done during drilling, producing, or during the abandonment process. A unique bench-scale physical model was used to simulate expansion of a previously-cemented casing under field-like conditions. Experimental measurements obtained during low-percentage casing expansion exhibited improvement of cement integrity with significant changes in the cement sheath. Successful multi-rate flow-through experiments with nitrogen gas showed the effectiveness of this technique in complete closure of microannular gas leakage pathways, providing ideal cement remediation. If implemented, this technology has potential to become a cement remediation technique for leaks behind the casing.
Operators set cement plugs for a variety of reasons: abandonment, sidetracking, lost-circulation control, or remedial work. However, as important as plug cementing can be to the overall success of a well program, the process often is performed without regard to wellbore conditions. The additional rig time and material costs quickly add up as mUltiple plugs are set. The industry average is approximately 2.4 attempts per kickoff.' Studies have highlighted many of the problems encountered in plug cementing. Implementation of systematic, controlled procedures has established a near 100% success rate where used for establishing kickoff plugs. Recommended Plug ProcedureRecent work has determined that cement plugs require the same degree of planning as primary cement jobs, plus special considerations for plug stability. ',2 Ajoint team of operators and a cementing service company evaluated the problems and mechanics of plug cementing. The team developed a recommended plug procedure (RPP) to alleviate problems. The procedure provides a reliable plugsetting procedure and develops a method to monitor success rates. The RPP addresses communication, temperature estimation, mud removal, slurry design, slurry volume, and plug stability. Understanding the relationships between these variables is vital to thc SIlCcess of spotting a balanced cement plug.
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