Kris Ravi, Robert Darbe - Halliburton
Sustained casing pressure (SCP) is a serious issue in many wells. The SCP occurs on the annulus of the completed wells when formation fluid repeatedly rebuilds pressure after the pressure has been bled to zero. SCP can be caused by tubing and casing leaks, poor primary cementing, or damage to the cement sheath resulting from thermal and pressure loading.
Two key mechanisms can solve the problem of SCP. The first part of the solution is a fit-for-purpose cement sheath with optimum mechanical properties (such as elasticity and tensile strength) that allow it to withstand cyclic loads and prevent fatigue failure. The second mechanism is an auto-seal feature in the cement sheath itself. The way this works is that when the cement sheath fails as a result of unexpected events and develops cracks or micro-annuli, the sheath can seal itself automatically when it is exposed to formation fluids like flowing gas or crude oil.
Extensive studies have been undertaken to analyze the effect of well operations on the cement sheath. Studies have also measured the properties of the cement sheath and have evaluated its auto sealing capability.
The problem
The annular pressure observed in some wells result in burst or collapsed casing and tubing, which in the extreme case can release toxic gases. The cost of well interventions required to manage sustained casing pressure is usually high and can be prohibitive for some assets, leading to well abandonment and loss of production.
The Bourgoyne report catalogues 11,000 casing strings in 8,000 wells in the Gulf of Mexico that showed sustained casing pressure in the late 90s. From a cementing point of view, this could be the result of the cement slurry not having been placed in the entire annulus and/or the inability of the cement sheath to withstand stresses from well operations.
The auto-sealing capability is illustrated in the experimental setup where pressure can be applied to both the annular space and also the inner-pipe. Cement slurry is placed in the annulus and is allowed to set up while applying pressure to the annulus and holding the inner pipe at a higher pressure. Upon curing of the cement slurry, the inner-pipe pressure is taken to atmospheric conditions resulting in a micro-annulus.
Well operations include pressure testing, completions, hydraulic stimulation, production, and injection. These operations can change the pressure and temperature of the cement sheath from what it was when it was placed and cured in the annulus. The cement sheath should be able to withstand changes in pressure and temperature to help prevent damage and the subsequent loss of zonal isolation.
Preventing cement sheath damage becomes even more critical in wells with reservoirs that contain CO2 and those into which CO2 is injected. In the presence of water, CO2 reacts with neat Portland-based cement systems. Cracks and micro-annuli in the cement sheath allow paths that can enlarge as wet CO2 continuously flows past or through the cement sheath. The resulting wear accelerates cement sheath degradation.
Cyclic loads
The change in pressure and temperature that stresses the cement sheath is not a one-time occurrence. During the life of a well, change occur repeatedly as different well operations are carried out. A cement sheath should be constructed to withstand cycles of changes in pressure and temperature to help prevent cement sheath failure. The cement sheath should also be designed to help prevent fatigue failure from repeated loads.
A change in pressure and temperature should not stress the cement sheath to anything close to its failure limit. If a cement sheath is loaded close to its failure limits, the number of subsequent loads that it can withstand decreases. The S-N (stress-cycle) curve for metals, which evaluates the number of cycles to failure, clearly demonstrates that lower stresses result in a greater number of cycles required to cause fatigue failures. The stress on the cement sheath from each well operation should be low enough that it is able to withstand a large number of cycles without failure.
Analysis
The integrity of the cement sheath was analyzed during different well operations such as curing, pressure testing, stimulation, production, and injection. The changes in near wellbore stresses as the well is drilled were modeled, yielding a realistic initial condition when the cement slurry is placed in the annulus. This step is important in analyzing the cement sheath’s integrity. A software program evaluates the effect of different parameters such as well deviation, casing eccentricity, anisotropic stresses in the formation, and shear failure in the formation.
Cement sheath properties
The mechanical properties of the cement sheath - such as Young’s modulus, Poisson ratio, and plasticity parameters - are calculated by testing the cement sheath in a tri-axial cell under confined and unconfined conditions. The tensile strength is measured using the Brazilian method and the direct pull method.
Another important property is shrinkage as the cement slurry is curing. Shrinkage affects the initial stresses in the cement sheath and the sheath’s ability to withstand well operations during the life of the well.
Auto-sealing cement sheath
The primary mechanism by which zonal isolation is secured and maintained is the cement system in the annulus that can withstand well operations during the life of the well. Drilling fluid should be displaced, and the cement slurry should be placed in the entire annulus. The fit-for-purpose cement sheath should withstand well operations and prevent failure in all the possible modes such as cracking, de-bonding, and shear.
Unexpected events can cause the cement sheath to crack or develop a micro-annulus. In such circumstances, the secondary mechanism helps seal the cracks and micro-annuli automatically if the hydrocarbon attempts to flow through or past the cement sheath.
The cement sheath’s auto-sealing capability is deployed automatically and is the smart feature of the cement sheath.
Laboratory experiments
In one laboratory example, a set of experimental dimensions created a micro-annulus of 200 μm. In both gas and oil wells a micro-annulus of this size could easily be created during well operations. Such a failure can occur when a high density displacement fluid is swapped with sea water during completions, decreasing the pressure inside the casing significantly enough to create a micro-annulus. A micro-annulus also could be formed when a lower temperature fluid is injected inside the casing.
In the laboratory experiment, the gas from the storage cylinder was allowed to enter the annulus with a supply pressure. The gas flowed as long as the micro-annulus existed and was quantified by a gas flow meter located downstream of the test apparatus. When flow measurements were not being obtained, the test apparatus was shut in to allow pressure to equilibrate.
The gas flow rate is observed to rapidly decrease in the first 7 days with a 75% reduction in rate.
The results of the gas flow experiment, using gas that was predominantly methane, indicated a rapid decrease in gas flow rate in the first seven days with a 75% reduction in rate. At approximately 30 days after the micro-annulus was formed and subsequently exposed to gas, the rate decreased substantially. The decrease in flow rate is directly related to auto-sealing characteristics of the cement sheath.
Liquid hydrocarbon expansion
Similar cement expansion took place in the presence of liquid hydrocarbon. Experiments were carried out in a conventional ring mold test apparatus. The cement slurry was cured in a water bath for seven days; then the ring mold was placed in the autoclave surrounded by liquid hydrocarbon (diesel in the experiment). Expansion was measured at different time intervals. Expansion in the presence of hydrocarbon at the end of 225 days was about 3%. During the same time, expansion in water was about 0.2%
The tests indicated that the cement sheath can be designed to swell and expand in the presence of reservoir fluids such as methane gas and liquid hydrocarbon. The auto-sealing capability should close micro-annuli of the order of 100-250 microns. This level of expansion should prevent the flow of fluids through the annulus as well as sustained casing pressure.
Field implementation
This two-mechanism solution to SCP has the potential to change the established paradigm in well cementing and achieve improved well integrity. The solution has been implemented around the globe in a number of fields. A few examples of where this solution has been implemented are South Texas, Gulf of Mexico, Asia, Europe, and Middle East. These wells are in production and are performing without any annular pressure. The fit-for-purpose cement slurries help maximize the value of the asset and support health, safety, and environment (HSE) requirements.
