| Oilfield Technology Reprinted from January 2016

Tesco Corporation’s tubular services segment is focused on enhancing the cementing process by providing a technology offering and recommendations for improved

well integrity and production levels compared to conventional cementing methods. Specifically, operators are being encouraged to implement a valuable well-cementing technique referred to as dynamic cementation that provides these benefits, while also resulting in lower operating costs.

Oil and gas production can be adversely affected by poor zonal isolation due to incomplete drilling fluid displacement, especially in deviated wells. Under-displaced mud, also known as partially dehydrated gelled (PDG) drilling fluid, is highly detrimental to well integrity. Conventional well cementing techniques do not adequately remove PDG and do not allow for optimal cement bonding. This is a problem that has affected drillers for many years. Conventional well-cementing techniques also have a tendency to allow for fluid to become trapped in and around the casing, thus creating an environment where gases can migrate along the length of a cased hole. This phenomenon has been confirmed using computational fluid dynamics (CFD) software and field experiments. Unfortunately, the well construction process only allows for one chance to build a reliable primary cementing system. Failure to perform a cementing job correctly the first time can lead to remedial

cementing work which is costly and is unlikely to produce identical results as a well-executed primary cement job.

There are many methods currently available that can improve the overall quality of a cementing job and help prevent the need for expensive remedial work. Such methods include proper hole cleaning, casing centralisation and casing movement during cementation. Despite an industry acceptance of the value of continuous pipe movement, the ability to achieve this operationally has proven difficult. The dynamic cementation process has allowed many operators in North American land operations and the Gulf of Mexico to achieve this, especially when met with the challenge of providing safer and more reliable cementing. Fernando Assing, CEO of Tesco Corporation said: “Typical tubular services providers are not often involved nor do they participate in this very important part of the operation. Tesco is now involved in this process and is currently providing technology solutions that allow operators to mitigate these issues. The company’s technologies enable rotation and reciprocation during the cementing operation and also significantly reduce transition time from casing-on-bottom to cementing. This enhances the cement job and provides the operator an opportunity to increase value while reducing costs and risks at the same time.”

Conrad Ojong Ojong, Tesco Corporation, USA, compares

the dynamic cementing technique to conventional well cementing methods.

PROVEN BENEFITS OF DYNAMIC CEMENTING

Reprinted from January 2016 Oilfield Technology |

Defining the problemGas migration may occur when formation pressure is higher than annular pressure (Figure 1). This problem can lead to an uncontrolled release of hydrocarbons to surface. Contributing factors such as poor mud filter cake removal, premature gelation and inadequate interfacial bonding can allow gas to migrate through the cement sheath or micro-annulus. The company provides a solution for the premature gelation which can occur during extensive nonproductive time (NPT) during rig up of conventional cementing equipment. During this time period, circulation is stopped and gelation of thixotropic drilling fluids can occur, leading to fluid channelling and cement losses (Figure 2).

Another negative effect of early gelation is that it causes the slurry to become load bearing. When part or the entire cement column becomes load bearing, hydrostatic pressure loss can occur, resulting in an underbalanced annulus, which allows for early gas migration.

Often, a potential problem with cement integrity is poor casing placement. In a perfect scenario, casing would be perfectly centred in the annulus in both static and dynamic states. The measure of casing centralisation in the annulus is called stand-off (Figure 4). Due to irregularities or tortuosity of the wellbore, pipe is not properly centred and requires centralisers to increase stand-off. When casing lies against the side of the wellbore, it can significantly impact the flow of cement around the casing and create irregular cement wall thickness (Figure 5).

Proposing the solutionDynamic cementation is accomplished by rotating and reciprocating the casing while cement is being pumped. The continuous pipe movement increases the effective volume of drilling fluid moving in the annulus. The additional continuous movement reduces the surface tension between the drilling fluids and the casing and between the drilling fluids and the formation wall. It also helps to break up cutting bridges that can occur and allows for drilling cuttings and drilling fluids to be pumped more easily out of the wellbore.

Continuous rotation of the casing string reduces the effects of early gelation, ensuring an overbalanced annulus, thereby mitigating early gas migration. To make continuous rotation possible, the cement swivel is placed between the casing running tool (CRT) and the top drive. The cement swivel performs the same function as a conventional cement head, except it has the additional benefit of allowing an operator to commence cementing operations almost immediately after the casing has been run. This saves the rig hours of time that traditionally would have been spent rigging up a conventional cement head system. As the cement swivel is fully rigged up to the CRT, the casing drive system (CDS), before arrival on site, there is no additional rig up time required for the sub (Figure 6). An anti-rotating bracket (ARB) is used to hold the swivel body secure and prevent rotation of the sub during operation. The cement line from the pump truck is connected to the side-entry port of the cement swivel.

Rotation of casing also ensures the cement is circulated completely around the casing, thus reducing the chances for thin

cement sections and leading to better zonal isolation. Casing movement and good centralisation leads to the ultimate goal of properly centralised casing with full circumferential cementing (Figure 3).

Case studies and industry acceptance

Case study oneCanacol Energy, in conjunction with Tesco Corporation, has performed a number cementing jobs using dynamic cementation techniques. These joint projects were performed in Colombia

Figure 1. This graphic demonstrates the various ways in which fluid or gas migration can occur as a result of an inadequate cement job.

Figure 2. A cross section of casing and cement after a trial operation is performed without rotation during cementation. It is clear that almost 40% of the mud in the annulus was not displaced due to gel strength and dehydration. Rotation during cementation would have kept the mud moving by exerting mechanical energy downhole to break the gel strength.

| Oilfield Technology Reprinted from January 2016

within the Llanos basin of the Casanare territory. Canacol wanted to improve the overall cement integrity of wells being drilled, which were directional with an average of 22˚ deviation in their 8 ½ in. and 7 in. sections. The new dynamic cementation technique was applied to more than 15 wells with highly satisfactory results.

Case study twoDynamic cementation was employed in the Eastern Hemisphere for the first time in 2014. Due to concerns over oil price instability and high drilling costs, the operator was challenged to find new ways to reduce costs. The company highlighted rig NPT associated with slow transition time between traditional casing running and cementing. This is a recurring source of NPT in the Eastern Hemisphere as transition times can take anywhere between two to three hours on land and three to five hours offshore. Dynamic cementation techniques were able to greatly reduce the NPT by allowing for a much quicker transition from casing running to cementing.

Another costly process uncovered in the assessment was the remedial work that was required after a poor quality cement job, typical of traditional casing and cementing practices. Computational fluid dynamics (CFD) simulation and 3D cement bond log-variable density log (CBL-VDL) charts indicated a mud displacement efficiency of approximately 50 - 60% on these wells. The ultrasonic images (USI) revealed mud channelling, irregular CBL amplitude and weak formation signal. All possible indicators of issues that could affect the overall zonal isolation and the integrity of the set-cement sheath.

Dynamic cementation practices were introduced to address the poor cementing issues that are seen in the industry today. A CDS tool and cement swivel were utilised to rotate the casing strings at 20 rpm and reciprocate at 4 ft/min during cementing operations. Analysis of the resulting USI and 3D CBL-VDL charts showed optimal isolation, high-quality, well-bonded cement and virtually no trace of unwanted fluids. In addition, the CBL amplitude was low and flat while VDL indicated strong formation signal.

Collecting field results from cases such as these have contributed to the growing body of knowledge on cementing issues and potential solutions. Dynamic cementation allows the operator to reduce unnecessary NPT during operational transitions and to avoid the need for costly remedial cementing. This increases rig efficiencies and produces higher quality cementation, ensuring the long term economic viability of the well.

References1. Holt, C., Lahoti, N., ‘Dynamic Cementation Improves Wellbore Construction

and Reduces the Hazards of Groundwater Contamination in Shale Plays’ SPE-Canadian unconventional Resources Conference, Calgary Alberta, (2012).

Figure 5. Displacement efficiency is the degree or ability of one fluid to displace another. This can be calculated as: displacement efficiency = current area/ total annular area = cemented area + displaced area.

Figure 4. Poor stand-off leads to poor mud displacement efficiency and poor zonal isolation. Stand-off = C/A-B, when A = wellbore diameter, B = pipe diameter, C = shortest distance between the pipe and the wellbore wall. In a perfect scenario, when the casing is at the centre, A-B = C and standoff = 1 (100%).

Figure 3. Pipe movement during cement placement aids in more evenly displacing mud, which can otherwise become trapped on the narrow side of the eccentric annulus.

Figure 6. Tesco cement solution : the Casing Drive System™, cement swivel, ball launcher and TesTork.