With the increasing global demand for energy, there is a need for energy companies to search for resources in more challenging environments. In order to accomplish these enterprises, the use of new technologies is essential in increasing productivity and reducing costs.
The latest oil and gas discoveries in the pre-salt layer offshore Brazil present a great opportunity for future development. There are several technological and logistical challenges that are being considered by the industry to develop new families of subsea devices to support the service requirements.
The pre-salt layer is a large oil and gas reservoir located in the Santos Basin/SP, Campos Basin/RJ and Espírito Santo Basin/ES. These reservoirs are located under the salt layer, which can be up to 2 km thick and lies 5 – 7 km below sea level.
These reserves were formed approximately 100 million years ago, from the decomposition of organic materials. Technicians have not yet been able to estimate the total amount of oil and gas contained in the pre-salt layer, but Brazil could become one of the largest oil producers and exporters in the world. However, investment costs are expected to be very high due to the technical challenges associated with the subsea depth and geological configuration of the reservoir. It is believed that only around 2016 these reserves are being exploited on a large scale.
Offshore production is complex due to the numerous variables involved and the requirements for reaching deep and ultra-deepwater, pre-salt petroleum with aggressive fluid characteristics, fields in remote areas and other environmental issues. The selection of the most suitable field development strategies and production system for a given scenario depends on the field characteristics such as geographical location, water depth, environmental conditions, as well as previous knowledge based on similar systems already in use for oil and gas production. Elements of a field production system include different types of production platforms, mooring systems, subsea equipments, reservoir main characteristics, wells and lifting processes.
Ultra deep well construction
One of the technological challenges of pre-salt exploration is the well construction process, in which drilling and cementing through layers of ultra deepwater, and particularly challenging sediments and salt layers is required. For example, to reach the oil reservoir in the Campos Basin drilling operations involve drilling and cementing through two layers. The first layer, water, can be up to 2 km deep. The second layer, which can be up to 7 km deep, presents additional challenges such as: high temperatures ranging from 80 – 150 °C, high pressures and corrosive gases such as H2S and CO2.
The well construction process includes drilling a borehole, lowering a steel pipe (casing) through the borehole and placing a cement sheath in the annulus between the casing and the formation. Primary cementing is a critical procedure in the well construction process. The cement sheath provides a hydraulic seal that establishes zonal isolation, preventing fluid communication between producing zones in the borehole and blocking the escape of fluids to the surface. The cement sheath also anchors and supports the casing string and protects the steel casing against corrosion by formation fluids. Failure to achieve these objectives may severely limit the well’s ability to reach its full production potential.1
Parameters to consider when designing a cement slurry include:
- Vertical well depth.
- Well angle.
- Casing dimensions.
- Formation temperature.
- Pore pressure.
- Fracture pressure.
- Hydrostatic pressure. (The pressure at any point in a column of fluid caused by the weight of fluid above that point. Controlling the hydrostatic pressure of a mud column is a critical part of mud engineering).
The differential pressure between the pore pressure and the fracture pressure, also known as ‘operational window’, will determine the specific mass of the cement slurry. If the bottomhole pressure due to the weight of cement slurry column plus frictional losses is greater than the pore pressure, or exceeds the rock strength, fractures in the rock will open, and cement will be lost to the formation. On the other hand, if the bottomhole pressure is smaller than the pore pressure formation fluids, including oil or gas, will enter the well, which can lead to blowouts. Both scenarios are to be considered when designing a cement slurry in order to prevent undesirable outcomes such as formation damage, which could compromise the well productivity, and/or accidents on the rig. Drilling and cementing through weaker formations and depleted zones requires the use of lightweight cements to line, seal and reinforce the borehole. Lightweight cement slurries can be achieved by water extension, foam cement or lightweight microspheres. Advantages associated with the use of lightweight microspheres include the simplicity of operations and a reduced equipment and resources footprint, which is always an advantage offshore or at remote inland locations.
Reduced density cement slurries
3M™ Glass Bubbles HGS Series are engineered hollow glass spheres that can used as a density reducing agent for oil and gas drilling fluids and cements. The spherical shape of these glass bubbles offer a number of important benefits, including lower viscosity and improved flow. The chemically-stable soda-lime-borosilicate glass composition of the bubbles makes them virtually insoluble in water or oil. Their low alkalinity gives them compatibility with most resins, stable viscosity and long shelf life. They are also non-compressible.
The bubbles are formulated for a high strength-to-weight ratio. This allows greater survivability under demanding downhole conditions. They are available in varying densities and crush strengths to help meet specific downhole conditions.
The low-density and high compressive strength cements made with glass bubbles can offer a number of advantages over other density-reducing materials such as nitrogen foam and cenospheres:
- Improved production: helps wells produce at their maximum capacity, and keep on producing over a longer period of time.
- Less remediation: helps strengthen cements that can mean less remediation due to cement failures. According to Cementing Solutions, Inc., 15% of new wells need to be fixed before they are put into service because of cementing failures.
- Lower cost over the life of the well: cements made with glass bubbles offer improved durability to the stresses of pressure and temperature cycles, and can help lower costs over the life of the well.
- Helps eliminate multi-stage cementing, saving time and labour costs.
- Reduced formation damage, helping keep product flowing at optimal rates.
- Improved bonding of the cement to the strata and the casing – fewer cement failures, less remediation.
- Better zonal isolation, preventing permeation of gases and fluid migration.
- Ease-of-use: the glass bubbles HGS series mix into the cement slurry more easily, quickly and uniformly. The resulting slurry can then be used to pour the well in a single-phase rather than a multi-phase process, with more precise density control.
- Less guesswork: helps service companies achieve better quality lightweight cement with more predictable results and less rework.
- Simplified formulation: with just one additive, service companies can design a variety of cements to meet their specific well conditions (e.g. pressure, temperature, formation).
- Easily transportable: glass bubbles are well suited for formulating high performance cements in remote, hard-to-supply areas, using semi-skilled personnel able to work with conventional equipment.
- Versatile: with the glass bubbles, cements can easily be formulated to achieve desired properties, such as low density, temperature resistance, etc.
- Safe handling: glass bubbles can be easily and safely blended with other materials, using standard mixing equipment, avoiding the need to use pressurised equipment such as nitrogen tanks.
Comparing the compressive strength between the different kinds of technologies used to reduce the density of cement slurries, it is noticeable that these bubbles have the greater range of use due to their low density and high resistance. Only normal and increased weight cements have more compressive strength.
In Brazil, the operators estimate that approximately 20% to 30% of offshore well operations require the use of lightweight cement (slurries that have density lower than 12.5 lb/gal.). The glass bubbles have been used in Campos Basin/RJ as a reducing density additive material for cementing operations. For offshore cement applications the most used microspheres, in Brazil, are HGS10000 and HGS18000 due to its high pressure resistance and its light weight.
- Nelson, E. B. ‘Well Cementing Fundamentals’, Oilfield Review Summer 2012:24, No.2.
Written by Hector T. B. Medeiros, Sergio E.A. Filho, Clara E. Mata, and Mikaella B. Oliveira, 3M. This is an abridged article from the June 2014 issue of Oilfield Technology, World Cement’s sister publication. To download the full issue of Oilfield Technology, register for free here.
Read the article online at: https://www.worldcement.com/special-reports/16062014/forever_blowing_bubbles_360/