New Interactive Cementing System Improves Zonal Isolation in Horizontal Wells
Hydraulic isolation of wells drilled with nonaqueous fluids (NAFs) relies heavily on eliminating mud from the annuli before placing cement.
Hydraulic isolation of wells drilled with nonaqueous fluids (NAFs) relies heavily on eliminating mud from the annuli before placing cement. Failure to expel all NAFs results in residual fluid channels that may compromise well integrity and can serve as nonproductive communication pathways during subsequent stimulation treatments. This paper presents an interactive cementing system (ICS) that is designed to mitigate these risks by reducing conductivity of the residual mud channels.
Currently, most horizontal wells drilled in the US are completed with multistage stimulation treatments. Isolation between stages is required both inside and outside the production casing string for effective stimulation. Although plugs can be used between stages for effective isolation inside the casing, cement must create external isolation. Lateral cement jobs are pumped according to plan; however, it is difficult to optimize fully all design inputs needed for an optimal cement job. With few operators logging horizontal well sections to understand actual lateral cement quality, mud channels are frequently identified as the root cause of stimulation inefficiency. Because completion effectiveness and well productivity depend upon good zonal isolation, profitability of the well is thought to be compromised.
Mud removal remains an integral part of the cementing process. A cement formulation was developed to improve zonal isolation in the case of poor mud removal. The ICS reacts with the hydrocarbons present in NAFs, reducing channel permeability and mobility to significantly improve the likelihood of hydraulic isolation. Specialized testing protocols were developed to demonstrate the capabilities of this new system. Additionally, American Petroleum Institute (API) testing methods and analytical techniques were used to optimize the slurry.
Historically, zonal isolation was dependent on complete mud removal, which was a function of many parameters. Even after well preparation is completed, mud removal is dependent on adequate centralization. Although most drilling engineers agree that well-centralized casing results in a better cement job, the tradeoff comes with the challenges of getting casing to bottom on long-lateral and extended-reach wells. Rotating and reciprocating casing can improve mud removal and cement quality, but significant cost and safety concerns limit their use. Spacer type and volume also influence mud removal. After decisions have been made on these cement-design parameters, cement placement should be optimized using computer simulation models to maintain adequate rheological parameters and achieve mud removal. The likelihood of a mud channel is much greater if these design parameters are not optimized.
After decades of trying to improve cement quality by optimizing the design criteria discussed previously, a new approach was needed. The ICS was developed on the basis of conversion of residual NAF to a substance that provides isolation. The system enhances zonal isolation in sections of the well where NAF remained after the cement job. The development of the new cement system focused on optimizing the active component concentration to provide a favorable interaction with NAF and, at the same time, minimize the effect on cement rheology and mechanical properties. Procedures developed in house demonstrated that the new system effectively reduces hydraulic conductivity of microannuli and channels up to several tenths of an inch in size.
Laboratory tests were designed to measure the ability of the ICS to alter NAF. The measured values were used to estimate how much pressure could be tolerated during a typical stimulation treatment, and initial results indicated the additive could fully isolate the external wellbore between stages. A Permian Basin operator agreed to test the new additive on five 10,000-ft-lateral horizontal wells. Cement bond logs (CBLs) run to identify the top of cement evaluated a small portion of the production cement job from the top of the curve to a few hundred feet above the kickoff point. A review of the CBLs to determine if the NAF transformation was measurable left no doubt that there was, in fact, a transformation of fluid, and the results complemented the laboratory testing.
Most exciting is the prospect of applying the ICS to other portions of a wellbore drilled with NAF. Wells need isolation for reasons far beyond fracture-stage isolation. Plans exist to perform additional tests to qualify other possible applications.
Acoustic CBL Results
CBL results from wells with the ICS are encouraging and indicate a potential step change in bond quality. The ICS was added to the production cement slurry for an initial five-well trial. The additive was placed in one well only in the horizontal section, but the ICS was added to the remaining four wells in the slurry up to the top of cement. Planning for the top of cement 500 ft inside the intermediate shoe is standard across all wells.
The beneficial effect of the ICS is immediately apparent in a side-by-side graphic of CBL amplitudes for the baseline and five trial wells. During the laboratory test, the force required to displace the high-yield-strength material was measured. This enabled the CBL logs to be interpreted with confidence that the additive provided isolation and did not simply improve CBL response. These results are discussed in the complete paper.
ICS is a stimuli-responsive cement platform with simple design rules to eliminate poor zonal isolation in cemented wellbores drilled with NAF. The ICS contains a polymeric material that is dry blended with the cement before pumping. After cement is in place, the ICS interacts with residual NAF, causing a marked rheological change; increasing the yield point of the NAF in a mud channel up to 20 times; and holding a differential pressure of 1,000 psi per 10 ft, depending on channel geometry. This rheological transformation occurs before the slurry reaches ultimate compressive strength and enables an NAF mud channel to withstand high differential pressures, providing a major benefit during fracturing operations (Fig. 1). It is possible to incorporate this additive into existing cement slurries without any significant change to the slurries’ original target properties, and jobs are pumped without any decrease in rate.
Implementation of the ICS during a stimulation treatment, which represents the highest pressure to which the cement sheath and the altered channel could possibly be exposed, verified that the altered channel would be able to hold the pressure difference between treated stages. Such enhanced hydraulic isolation will help avoid fracture initiation in lower formation stresses only, will reduce tortuosity and associated high fracture-initiation pressures, and will avoid communication between stages.
Taking into account the challenging environment of the horizontal-well-cementing process, the ICS is a good mitigation measure for wellbores with residual mud. However, it cannot be considered as a substitute for mud removal; thus, basic mud-removal best practices should be followed even when this technology is applied.
The complete paper provides a detailed discussion of the validation work flow for the ICS concept. The discussion includes API testing, NAF yield-strength change, scaled pressure testing, and field testing. In API testing, a set of tests was performed to make sure the polymeric material did not change the existing slurry and set cement properties in the concentrations used. With regard to NAF yield-strength change, evidence showed that NAF movability was changed after contact with the ICS. Scaled pressure testing demonstrated the ability of the ICS to alter the property of the NAF in mud channels and quantified the pressure it could hold.
The technical objectives for the field-test campaign included
- Verification of the system’s specifications for density and temperature ranges
- Evaluation of operational usability of the cement system
- Verification of the effect on cementing job quality, such as improvement in CBLs compared with those in the offset wells
As of the writing of the paper, eight successful field-test jobs had been performed using the new ICS technology. The paper focuses on the field-test campaign of five wells for a Permian operator. Two baseline wells were compared with four wells in which ICS was placed in both horizontal and vertical sections. The baseline wells did not have ICS placed in the vertical section. During the field trial, the wells with ICS were evaluated using CBL and variable-density logs (VDLs) on cable.
The wells were pumped with similar designs and slurries. All six wells had similar design and execution. Wells were circulated with the same diesel-based mud with a density of 9.3 lbm/gal before cementing. Openhole size was 6¾ in. with 5½-in. casing. The centralization-placement program assumed one centralizer per joint throughout the entire open hole, which gave up to 68% standoff at centralizers and fell to 0% of standoff between centralizers.
All six wells were executed without service-quality issues, plugs were bumped, and floats held. A review of the CBLs and VDLs for the five trial wells showed the beneficial effect of the ICS.
With more field tests to be conducted and other methods of evaluation, including the ultrasonic imaging tool, to be used, early results indicated that the cement system had a positive effect on cement bonding. This, in combination with laboratory testing, suggests that the new ICS alters the NAF in a mud channel and potentially can improve hydraulic isolation of the well beyond the extent achievable by mud removal alone.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191561, “Effective Zonal Isolation in Horizontal Wells: Mitigating Negative Impact of Mud Channels,” by Petr Kolchanov, SPE, Dominic Perroni, SPE, Anatoly Medvedev, SPE, Yan Gao, Randy Tercero, SPE, Larry Todd, SPE, and Bernhard Lungwitz, SPE, Schlumberger, and Kenneth Cowan and William Turner, SPE, Occidental Petroleum, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed.