The paper reports on a feasibility study on post-cementing quality control using recorded data from Top Cement Pulsation (TCP). TCP is a novel technique for preventing gas flow after cementing. In this method small hydraulic pressure pulses (usually 100 psi.) are applied repeatedly at the surface into the casing annulus starting immediately after cement placement until the end of the cement transition time. The volumes of water pumped into the annulus and returned during TCP is monitored and converted to Top Cement Displacement Record (TCDR). These short duration pulses make the slurry columns shear and fluidize. The fluidization prevents under balance- a primary cause of gas migration after cementing. It had been postulated that TCDR might provide valuable information on the quality of cement setting in the annulus of the well. Consequently the record should some how be analyzed to determine fluid loss volume, initial/final position of top of cement and identify problems such as high fluid loss, bridging; identify presence of high temperature zones. The paper presents a diagnostic method based on recognition of TCDR patterns. From mathematical modeling of pulse transmission, different characteristic patterns have been derived. In the method the TCDR recorded from actual TCP data is compared with the expected TCDR. Then, a difference in the TCDR pattern between the expected and the actual TCDR is analyzed. From the analysis it can be determined what might have happened to the fluids placed in the annulus. Also included in the paper is validation of the diagnostic technique using field data collected from wells subjected to TCP. The wells were having relatively long interval of open hole with cement top far below the previous casing shoe. From the analysis it was possible to verify the progressive shearing of fluid columns during each pulse, the rate of fluid loss, the zone of fluid loss, end of cement thickening period, and top of cement. Introduction It is very important that the cement pumped inside the annulus of a well maintain hydrostatic pressure during cement gelation. Also a strong seal created during cement setting could prevent annular gas migration and thus prevent problems throughout the life of the well. Top Cement Pulsation (TCP) is a novel technique for preventing gas flow after cementing. The basic concept of the TCP method is to apply relatively small pressure pulses (usually 100 psi) into the annulus from the surface at regular intervals starting immediately after cement placement until the end of the cement transition time. Haberman et al.1 proposed the TCP method in 1995. Since pulses are applied with only very short period between successive pulses, the slurry will not get enough time to regain the gel structure. These short duration pulses make the slurry columns shear and fluidize. The fluidization prevents under balance- a primary cause of gas migration after cementing. The volumes of water pumped into the annulus and returned during TCP is monitored and converted to Top Cement Displacement Record (TCDR). TCDR can provide valuable information on the quality of cement setting in the annulus of the well. The TCDR can be used to determine fluid loss volume, position of top of cement and identify the presence and top depth of high fluid loss, bridging, fluid influx, and high temperature zones.
Summary Because the use of adaptive drilling processes, such as managed pressure drilling (MPD), facilitate drilling of otherwise nondrillable wells with faster corrective action, the drilling industry should include the effect of gas dispersion, bubble suspension, fluid compressibility, and riser ballooning to avoid the overestimation of riser pressure and to produce more efficient well control methods. The IADC Deepwater Well Control Guideline recommends always addressing riser gas first, before proceeding to control the well in a well control situation. The intent is to remove the risk of gas reaching the surface and the rig floor, putting personnel and assets at risk. However, with the availability of equipment on the rig dedicated to handling riser gas and the fact that the riser is isolated from the wellbore, the atmosphere reduces the level of risk of gas in the riser, whereas the well below the subsea blowout preventer (SSBOP) poses a greater risk. In this paper, we discuss the results from full-scale experiments recently conducted in an extensively instrumented test well (LSU Well-2) and demonstrate that the riser pressures resulting from upward transport or aggregation of riser gas are much lower than the values estimated using the single-bubble model and industry thumb rules. We explain the danger of using an open-top riser top during the monitoring of gas-in-riser and how the situation can get out of control due to the potential dynamic unloading situation. Our research also demonstrates the minimal fluid bleedoff volumes required to reduce pressure buildup consequences of free gas migration in a fully closed riser containing low-compressibility liquid. A differential pressure methodology used in this paper for analysis was also used for detecting the presence, position, void fraction, and lead and tail velocity of the gas column in real time during each of the tests to make decisions during the tests. The results from a successful application of the fixed choke constant outflow (FCCO) method as a new method for circulating out gas from the riser by staying within the gas-handling capacity of the existing mud gas separator (MGS) on the rig are presented. This is the industry’s first test of the FCCO method.
Summary Conventional methods of managing gas-in-riser events are not optimal when the drilling riser is isolated from the formation by a closed subsea blowout preventer (BOP) on rigs equipped with mud gas separator (MGS), managed pressure drilling (MPD), or riser gas-handling equipment. The industry is concerned about exceeding the pressure limit of the riser and the flow capacity of the MGS and hence has not been able to reach a consensus on a circulation method for riser gas. This work is an analysis of the first-ever demonstration of the fixed-choke constant-outflow (FCCO) circulation method in synthetic-based mud (SBM) carried out in June 2022. The first-ever demonstration of the FCCO circulation method in water was performed in March 2021. The results from the water tests were discussed in IADC Gas-in-Riser/Riser Gas-Handling Subcommittee meetings, and the new fixed-choke method was named FCCO in November of 2021. The reason for using the acronym FCCO for the new method is that it allows the use of a fixed-choke opening percentage throughout the circulation period by managing the outflow and backpressure by varying only the pump rate. This work includes the comparison of the actual test results from the March 2021 FCCO tests in water with results estimated using a new model. This is followed by a discussion of the results from the June 2022 FCCO test in SBM. Nitrogen gas was injected into the bottom of an annulus 5,200-ft deep, vertical test well (9 5/8×2 7/8 in. casing/tubing) initially filled with water and instrumented with four downhole pressue and temperature gauges, and distributed fiber-optic sensors [distributed temperature sensing (DTS) and distributed acoustic sensing (DAS)] for water tests, and later filled with SBM. We started direct circulation to produce flow out of the annulus through a choke kept at a fixed open position (%) required for a preplanned applied surface backpressure (ASBP). We reduced pump rate if/as necessary to maintain this ASBP to ensure outflow rate within MGS flow capacity. We performed tests at different fixed-choke positions, different average ASBPs, and initial pump rates. We tested constant bottomhole pressure (CBHP) circulation and fixed pump rate methods also for comparison with the FCCO circulation method. The results from the FCCO tests demonstrated better control of outflow compared with the other methods. There is no need to use high ASBP. The use of a high ASBP suppressed the value of peak pressure. Installation of more than one gauge inside the riser enhances safety by allowing real-time influx detection capability, estimation of gas position, gas velocity, and gas void fraction. The FCCO method can be preplanned and easily substituted as the preferred circulation method for staying within the handling capacity of the existing MGS on the rig during gas-in-riser situations.
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