Bagheri, Mohammadreza (Research Centre for Fluid and Complex Systems, Coventry University) | Shariatipour, Seyed M. (Research Centre for Fluid and Complex Systems, Coventry University) | Ganjian, Eshmaiel (School of Energy, Construction and Environment, Built & Natural Environment Research Centre, Coventry University)
The fluid pressure, the stress due to the column of the cement in the annulus of oil and gas wells, and the radial pressure exerted on the cement sheath from the surrounding geological layers all affect the integrity of the cement sheath. This paper studies the impact of CO2-bearing fluids, coupled with the geomechanical alterations within the cement matrix on its integrity. These geochemical and geomechanical alterations within the cement matrix have been coupled to determine the cement lifespan. Two main scenarios including radial cracking and radial compaction, were assumed in order to investigate the behaviour of the cement matrix exposed to CO2-bearing fluids over long periods. If the radial pressure from the surrounding rocks on the cement matrix overcomes the strength of the degraded layers within the cement matrix, cement failure can be postponed, while on the other hand, high vertical stress on the cement matrix in the absence of a proper radial pressure can lead to a reduction in the cement lifespan. The radial cracking process generates local areas of high permeability around the outer face of the cement sheath. Our simulation results show at the shallower depths the cement matrices resist CO2-bearing fluids more and this delays exponentially the travel time of CO2-bearing fluids towards the Earth's surface. This is based on the evolution of CO2 gas from the aqueous phase due to the reduction in the fluid pressure at shallower depths, and consumption of CO2 in the reactions which occur at the deeper locations.
Operators in the North Sea have been concerned about the ability of the cement sheath to maintain sealing integrity because of the increasing number of reported failures in mature wells. This paper presents results from a new laboratory setup to visualize the source of issues. Many wells in the Cana-Woodford shale suffer from chronic sustained casing pressure (SCP) because of poor cement-sheath bonding. This work demonstrates cement design that includes evaluating cement-sheath mechanical integrity in intercalated salts.
The complete paper discusses the advancements in mud-displacement simulation that overcome the limitations of the previous-generation simulator and provide a more-realistic simulation in highly deviated and horizontal wells. Operators in the North Sea have been concerned about the ability of the cement sheath to maintain sealing integrity because of the increasing number of reported failures in mature wells. This paper presents results from a new laboratory setup to visualize the source of issues. Many wells in the Cana-Woodford shale suffer from chronic sustained casing pressure (SCP) because of poor cement-sheath bonding. As deeper and more complex well designs proliferate throughout oil and gas fields, well completion methods are challenged and new technologies are emerging to ensure safe, cost-efficient, and optimized completions.
Wells often end up producing oil and gas far longer than expected. But that often requires the operator to commit to building and maintaining facilities for the long haul. Archer’s Stronghold Barricade well decommissioning system leaves the casing in place, while perforating, washing, and cementing the annulus to create a cement barrier in a single trip. The combination of ultrasonic pulse-echo and flexural-attenuation measurements was adopted in this project in the South China Sea for cement-integrity evaluation.
The data collected via monitoring and metering applications are increasingly viewed as central to assessing production performance and in decision making to optimize field development and operations. Intervention and workover operations can significantly affect the structural integrity and fatigue life of subsea-wellhead systems. Methodologies for wellhead-fatigue analysis have improved, but have yet to account for thermal effects along the well. Moving to higher-capacity wellhead systems for high-pressure and high-temperature (HP/HT) environments will require a larger mandrel and conductor-casing size.
Well RXY is located in Cairn’s Ravva offshore field in the Krishna-Godavari Basin in India. One goal for the field was significant crude production by means of a secondary reservoir section. This paper compares the results of gas identification and lithology identification using pulsed-neutron spectroscopy in openhole and casedhole environments. Acquiring data from an abandoned subsea well has been done before, but never quite like this. As I read through the abstracts and papers that have been presented in the past year, I notice several key themes: verification of cement placement, development of new materials as a barrier, development of new additives to improve the cement barrier, and enhancement of existing techniques.
Gupta, Arpit (Weatherford Oil Tools M.E. Ltd) | Thomas, Emil (Weatherford Oil Tools M.E. Ltd) | Tomar, Gaurav (Cairn, Oil & Gas Vertical of Vedanta Limited) | Rawat, Ishita (Cairn, Oil & Gas Vertical of Vedanta Limited) | Prakash, Aditya (Cairn, Oil & Gas Vertical of Vedanta Limited) | Golwalkar, Anirudh (Cairn, Oil & Gas Vertical of Vedanta Limited) | Vermani, Sanjeev (Cairn, Oil & Gas Vertical of Vedanta Limited)
In offshore platforms, with high well density, slot recovery technique is an efficient way to target new / un-swept avenues to boost the production levels in a mature field. This leads to utilization of an appreciable length of parent bore which is an advantage to the operators globally in terms of surface facility retention and associated rig time saved. This paper discusses an actual case-study wherein dual casing exit was achieved in an offshore platform well resulting in significant time and cost savings.
For the subject well the subsurface targets were quite far from the mother-bore, resulting in a plan to side-track the well at a shallow depth where double casing existed, i.e. 9-5/8″ × 13-3/8″. The options available were pilot milling and dual exit using whipstock. Unlike multi-casing exits, pilot milling is a time consuming method which requires multiple trips and involves large volume of metal swarf handling at surface. The CBL-VDL verified the presence of cement outside 9 5/8″ casing that further supported the case of dual casing exit operation. Consequently, associated risks were discussed and plans to mitigate the same were put in place.
Single-trip 8-1/2″ whipstock-milling system was used to cut a window suitable for running drilling BHAs, liner, and completion equipment. The 9-5/8″ × 13-3/8″ annulus was monitored during milling and FIT test to check for any pressure communications. For well control scenario, arrangements were made for connecting the annulus to the choke manifold to ensure a closed system and thereby have provision of circulating through choke in case of gas migration in the 9-5/8″ × 13-3/8″ annulus. The window milling operation was done using sea water & intermittent Hi-vis sweeps. The window was milled successfully in a "single trip", thereby saving considerable rig time. No excess drag or held-up was observed and gauge loss on mills when pulled out of the hole was negligible. Well integrity was intact with no pressure communication in the annulus. The job was a successful one that led to finishing the well within the planned time and thereby, led to timely release of the jack up rig before the onset of adverse weather conditions.
Multi-casing exit technology in two or three casing strings opens the multi-level advantages to well intervention techniques especially in situations where the wells are old with limited access due to presence of fish or other restrictions that makes the deeper section of the well non-usable. Such sections can be avoided by sidetracking at a shallow depth and also provides an opportunity to access targets that are quite far from the original mother-bore.
Cement sheath is a critical barrier for maintaining well integrity. Formation of micro-annulus due to volume shrinkage and/or pressure/temperature changes is the major challenge in achieving good hydraulic seal. Expansion of cement after the placement is a promising solution to this problem. Expanding cement can potentially close micro-annulus and further achieve pre-stress condition because of the confinement. Primary aim of this paper is to investigate mechanical integrity of different pre-stressed cement system under loading condition.
To achieve the objectives, finite element modelling approach was employed. Three dimensional computer models consisting of liner, cement sheath, and casing were developed. Pre-stress condition was generated by modelling contact interference at the cement-casing interface. Three cement (ductile, moderately ductile, and brittle) were considered for simulation cases. Wellbore and annulus pressure were applied. Resultant, radial, hoop, and maximum shear stresses were investigated at the cement-pipe interface to assess mechanical integrity. For comparison purpose, similar simulations were conducted using cement sheath without pre-stress and cement system representing uniform volume shrinkage and presence micro-annulus.
For constant wellbore pressure, the radial stresses observed in all three types of cement system were practically similar and decreased as pre-stress was increased. Hoop stress also reduced with increase in compressive pre-load. However, their absolute values were distinct for different cement types. These results indicate that cement system with compressive pre-load can notably reduce the risk of radial crack failure by providing compensatory compressive stress. However, on the contrary, the maximum shear stress developed at cement-pipe interface, increased because of pre-load. This can compromise the mechanical integrity by reducing the safety margin on shear failure. Thus, the selection of expansive cement should be made after carefully weighing reduced risk of radial failure/debonding against the increased risks of shear failure.
This paper provides novel information on expanding cement from the perspective of mechanical stresses and integrity. Modelling approach discussed in this work, can be used to estimate amount of pre-stress required for a selected cement system under anticipated wellbore loads.