Laboratory tests conducted by Halliburton in their state-of-the-art facility in Duncan, Okla., demonstrate that the depth of carbonation is directly proportional to the permeability and chemical makeup of the cement sheath. Therefore, potential CO2-induced corrosion of the cement sheath in CCS wells is an area that needs to be evaluated and addressed. (Images courtesy of Halliburton)

The wells constructed to exploit the hydrocarbon and geothermal energy from these reservoirs also require sophisticated technologies that can both resist the potential corrosive effects of CO2 and H2S and provide structural and chemical integrity. Very often the CO2 produced from these wells is re-injected for the purpose of storage or enhanced oil recovery (EOR).

Environmental sustainability

Cementing operations are routine for both land-based and offshore energy wells (oil, gas, geothermal); the contribution of the cement sheath can be taken for granted.

Environmental sustainability requires a re-evaluation of the strategic role of a reliable cement sheath and how it provides a tremendous contribution toward helping increase the life of the well — which in turn minimizes the impact on the environment. It is important to note that the cement sheath supports the casing; so that the combined cement sheath plus casing provide structural integrity to withstand formation and overburden pressures across distances that can exceed 30,000 ft (9,150 m). And, the cement sheath is the only barrier providing an impermeable seal that protects the casing from corrosion — while at the same time shutting off zones with a pressure-tight lock that crimps off differential pressure zones and thief zones. The result can be long-term secure containment — and in this discussion, containment of CO2 away from the atmosphere. All of this is accomplished in a highly dynamic environment where pressures can fluctuate by as much as 6,000 to 10,000 psi and where temperatures can exceed 325°F (163°C) and also vary over time. A truly successful primary cement job design should include planning for what happens to the slurry after it is successfully pumped into the annulus.

CO2 is active

In normally occurring quantities CO2 is safe — so safe it is consumed regularly in carbonated beverages. However, CO2 is chemically active and CO2 gas in the presence of water (H2O) readily combines to produce carbonic acid (H2CO3) that when in contact with cement can slowly weaken it — thus compromising all of the aforementioned objectives of cementing. In some cases, the cement may lose enough strength to potentially impair secure confinement of CO2 when stored underground. Laboratory tests conducted by Halliburton in their state-of-the-art facility in Duncan, Okla., demonstrate that the depth of carbonation is directly proportional to the permeability and chemical makeup of the cement sheath. Therefore, potential CO2 -induced corrosion of the cement sheath in carbon capture and storage (CCS) wells is an area that needs to be evaluated and addressed.

Withstanding the test of time

Life of the well takes on a new meaning with CO2 storage wells that need to provide zonal isolation for decades. Being able to predict the performance of the cement sheath over time and the cumulative effect of stresses due to the changing variables of downhole conditions is imperative for engineering a cement sheath to retain integrity for the life of the well. Additionally, such analysis allows for the cement sheath to be tailored to the unique attributes of each well. Simulations based on finite element analysis (FEA) as leveraged through analytical tools provided by Halliburton are changing the industry’s fundamental approach to cementing. Conventional cementing procedures predominantly focus on well-construction activities and properties such as cement slurry placement, preventing gas migration and compressive strength development. This approach has been necessary to optimize drilling time — and when combined with FEA simulations based on fundamental variables and wellbore conditions, the resulting engineered solution optimizes zonal isolation for the life of the well.

Properties of the formation, casing, and cement components along with details of well operation can be analyzed using FEA to determine “remaining capacity of the cement sheath to withstand pressure/temperature cycles”. This is the capacity remaining in the cement sheath following the well operations. If the analysis shows that the remaining capacity is high, then the cement sheath is far from failure and is able to withstand the stresses from well operations. Additionally, analyzing other variables that contribute to preparing the well bore to receive cement slurry such as casing eccentricity, centralization, and mud displacement via computational fluid dynamics (CFD) — results in an engineered solution with a far higher probability of not only maintaining wellbore integrity but of even increasing the life of the well, which in turn helps maximize environmental protection. Halliburton achieves this with FEA and CFD tools such as WellLife service, OptiCem software and Displace 3-D cementing simulator.

CO2-resistant cement

It is well documented in industry literature that carbonation occurs when neat Portland cement is exposed to CO2. Halliburton has developed systems that have properties that are proven to better endure the impact of corrosive downhole environments. The company has created a set of fit-for-purpose solutions that allow each cement system to be designed specifically for any given set of wellbore conditions. One example is the company’s specially formulated CorrosaCem cement blends, which are engineered for long-term zonal isolation in particularly corrosive wellbore environments. These blends are designed to minimize the carbonation effect in CO2 wells. Laboratory core flow tests show that CorrosaCem CO2 cement can limit CO2 penetration to a shallow layer and instantly seal its permeability when exposed to CO2.

Case history

At the northeast end of the Caspian Sea (Kazakhstan) is a carbonate reservoir characterized by high pressure and high H2S and CO2 content. As published in SPE/IADC 119396 in 2009*, Halliburton was brought in by an operator to address the challenge of containing reservoir and injection fluids — a challenge made more acute due to high pore-pressures, varying properties of rock, and the corrosive fluids. The company used an integrated approach to preparing the well bore to receive cement slurry and engineering zonal isolation for the life of the well via life of the well analysis and optimized slurry design. As a result, a spacer fluid was designed to optimize surfactant concentration to effectively wet the surface to help optimize mud removal and slurry placement.

Additionally, cement slurry with the ability to react and respond was designed to withstand the well operations based on the well conditions known at the time of the simulation. Laboratory tests were run to determine the tensile strength, cohesion and friction angle as well as the ability of the cement sheath to expand in order to self heal should a micro-annuli or any cracks form in the sheath. The chemical integrity of the newly designed cement system when exposed to the corrosive fluids of Kashagan reservoir was evaluated in the laboratory. For comparison, a neat Portland cement sheath was also tested under the same conditions. The newly designed cement system maintained chemical integrity even after being exposed to the corrosive fluids for more than one year. In contrast, the neat Portland cement sheath showed clear evidence of being chemically attacked by the corrosive fluids. The solution was deployed successfully in six wells and the slurry design with improved elasticity, expansion characteristics, and CO2 resistance was evaluated by bond logs that were compared with the bond log from previous jobs run with conventional cementing techniques in the same field.

The results from all six of the wells cemented with the integrated approach showed dramatic improvement in the placement and bonding of the sheath to the casing and the formation. Subsequent well operations are being conducted on all these wells and are functioning without any problems. The wells in Kashagan field are being cemented using the new cement system and the procedure developed from the integrated approach.

*Nevio Moroni, C. Repetto, N. Dettori, ENI E&P Divisions, Kris Ravi, Halliburton 2009. Zonal Isolation Achieved in Kashagan Field through Integrated Approach. Paper SPE/IADC 119396 presented at the SPE/IADC Drilling Conference and Exhibition held in Amsterdam, The Netherlands, 17-19 March.