By Deborah Duckworth, BJ Services Co
Although cementing costs typically represent less than 10% of total planned wells costs for US onshore operations, failure to get good zonal isolation and adequate cement bonding to the casing and formation can be catastrophic, from loss of potential pay zones to the loss of the well.
Still, personnel accustomed to cementing operations may take them for granted. Often, the success of a cementing operation is measured on location with a single entry in the drilling report: Bumped the plug and floats held. Good cement job. This practice begs the question: Does bumping the plug and pumping all the cement actually reflect the efficiency and quality of a mission-critical operation?
A recent two-year cementing project in Wyoming provided an opportunity to develop truly useful metrics for evaluating cementing success. The effort not only improved cementing operations but also resulted in economic benefits to the operator and service company.
Figure 1: A two-year cementing project in
Wyoming involved careful measurement of
cementing value, resulting in economic benefits
to the operator and service company.
MEASURING CEMENT VALUE
The project objective was clear: provide fit-for-purpose zonal isolation for new development wells without negative health, safety and environmental (HSE) impact. The challenges included:
- Coordinating all phases of cementing activity for eight to nine drilling rigs (25 to 35 jobs/month).
- Adhering to strict environmental constraints, including seasonal wildlife stipulations limiting access to locations.
- Meeting economic objectives heavily impacted by Rocky Mountain market conditions, including gas prices and deliverability.
- Accommodating underbalanced drilling operations.
- Performing 24-hour simultaneous operations on multiple drilling pads (drilling, completions, facilities work, construction, production work and well servicing).
The service company had to find metrics that measured cementing performance and quantified real value to the operator. These key performance indicators (KPIs) were tracked and informally reviewed with the rig team upon completion of each job. Higher-level reviews were conducted quarterly to formally compare the most recent operations with past performance. This collaborative effort allowed service company and operator personnel to share experiences and explore ways to improve.
The key areas of measurement were operations, engineering and economics. These categories were closely integrated, and success or failure in one area affected the others. For example, arriving late for a job on a well on a restricted access pad resulted in increased NPT. The delay increased rig standby costs and extended the total drilling days for the well. The increased rig time on location delayed completions, which were conducted as batch operations to optimize the economics and allow the operator to finish all the wells on a pad before wildlife stipulations began. Thus, the KPIs were designed to capture not only the cost of the extra rig time but also the total “ripple effect” of a single event.
The main determiner of operational success was HSE compliance. A job could be completed as designed and within budget, but unplanned discharges, accidents and injuries nullified success.
The first step was to integrate the safety cultures of the operator and service company. Training deficiencies were identified and resolved before operations began. The first KPIs in the project tracked accidents, injuries and unplanned discharges, as well as the frequency and quality of job safety analyses (JSAs) developed for each operation. The service company participated in the operator’s spill team and region safety meetings and core contractor meetings. The experiences shared were then incorporated into the project.
Operational success depended on effective planning and communication. In managing cementing operations for multiple rigs, the service company had to develop operational efficiencies through resource management. Its in-house engineer attended daily drilling meetings, and the company assigned an operations field coordinator. These individuals communicated daily with the operator to provide service company operations managers with current information. They also communicated with each other at least once a day. This information sharing helped minimize surprises and NPT while maximizing resource efficiency. For example, cement could be loaded and tested in advance. If the rig reached casing point earlier than anticipated, the service company could quickly respond.
Another KPI was developed to track the time on location. A job timeline categorized all location time to identify operational inefficiencies (Figure 2).
Upon sharing the information among service company and operator personnel, average rig-up times were reduced 6%, service company NPT was reduced from a high of 17% to a current average of under 3%, and JSA time increased, indicating greater attention to HSE detail on location. The time spent on location waiting for rig operations also decreased. This allowed the service company to better manage resources. As an example, tracking the NPT associated with lab testing justified an increase in lab staffing and capital expenditure.
During the course of the project, well design constantly evolved to incorporate efficiencies. The service company was challenged to review and revise cementing designs to accommodate these changes and to provide solutions that would further increase efficiency. Each cement job was to be fit-for-purpose: The cementing program for each well required review and engineering, rather than designing one slurry for the field and adjusting volumes for each well.
For example, several water disposal wells were drilled in the field. The depths of these wells were similar to those of the shallow intermediate casing point in several of the production wells. Although both casing strings were cemented back to surface, the slurries used in the disposal wells were not the same as those used for the intermediates. The disposal wells required different cement properties because of the differences in the planned wellbore operations.
The other factor influencing slurry design was the practice of underbalanced drilling. The wellbores could encounter gas kicks and losses, which had to be controlled during drilling and addressed when cementing. If the service company could not maintain well control during its operations, then the job was poorly designed and implemented.
Figure 2: One project metric involved tracking
and categorizing all location time to identify
operational inefficiencies. “WO Service Co”
includes being late to location, hot-shotting
chemicals, equipment downtime and waiting
on lab tests.
The initial KPI for measuring engineering success on the long strings was the cement bond log (CBL). The CBL provided a visual representation of the effectiveness of the zonal isolation. If a CBL was deemed fair or poor, a post-job analysis was done to identify engineering weaknesses and areas for improvement. If necessary, slurry design changes were proposed, modeled and tested for use on offset wells in the area. A second KPI tracked the top of cement from the CBL compared with the designed TOC. A third KPI tracked the number of coiled tubing clean-outs performed to remove cement fill inside the casing in order to commence completion operations.
For surface casing jobs, there were no CBLs, so engineering success was tracked by tracking cement returns to surface. In more than 150 wells, fewer than 5% encountered losses or operational issues that prevented returns to surface and required remedial cementing to meet regulatory compliance. Most were the result of greater than normal washouts due to drilling issues or unanticipated loss zones. These were addressed in offsets by designing jobs with additional excess cement and optimized use of lost-circulation materials and low-density or ultra-lightweight slurries. Operational errors were critiqued and a root cause analysis and corrective action plan were developed. If the operational failure was on the part of the service company, another KPI tracked how quickly the service company reacted and implemented changes.
The engineering KPIs allowed the operator and service company to develop best practices that mitigated or eliminated engineering issues. For example, cement fill in the casing was investigated and discovered to be related to float equipment and cement plug compatibility. As a result, a “best practice” was developed requiring the use of compatible plugs and float equipment of like manufacture, as well as the use of high-efficiency plugs on long strings. The service company also reviewed operations and developed a standard practice for washing up after the cement job. This practice was included on every cementing recommendation.
Economic success would appear to be the easiest area to track and quantify. However, the project was a continuation of an established operation in which the obvious economic efficiencies had already been implemented.
The service company had to consider new opportunities for economizing. The economics KPIs tracked the variance between proposed and actual costs, total NPT attributed to service company operations, cementing costs per foot, dollar value of the new technology and surface casing top-outs. Table 1 shows the economic impact of these KPIs.
The greatest economic impact came from the initial redesign of the cement slurries. Because the wells were drilled underbalanced, the operator was cementing the production casing with foamed cement to control losses and minimize gas cutting of the cement slurry. The drawbacks of the design were HSE risks and the need for extra equipment on tight locations. After reviewing CBLs and job designs, the service company in-house engineer determined that conventional slurries at the same densities would provide the required performance. This change eliminated HSE concerns and extra equipment needs, and reduced the average job cost by 40%. As an additional bonus, the CBLs improved because the new design included gas migration additives and bonding agents instead of relying on foam properties for those functions.
Table 1: Economic key performance
indicators and impacts.
Another economic success involved using new technology to reduce behind-pipe losses of expensive, oil-based drilling mud. Recovering the mud and conditioning it for use on another well would reduce the total mud costs. The BJ engineer proposed using scavenger slurries – versatile, low-cost spacers – ahead of the cement. The spacer cost $50/bbl but allowed the mud to be recovered at a 1:1 ratio for a net savings to the operator of $75/bbl. This savings grew as the price of oil rose during the course of the project. (Even at today’s lower product prices, the operator sees a net gain by using scavenger slurries.)
A third economic impact was redesigning slurries to set up faster (pump time + 1 hour safety factor) and provide adequate compressive strength to resume drilling operations after 8 to 12 hours instead of 16 hours.
Finally, by actively tracking well progress and redesigning cement proposals to reflect actual well conditions, the actual costs ended up being an average 6.3% lower than planned costs. Effective resource management also improved operations efficiencies, thus reducing time on location and related costs.
An opportunity exists with every cement job to look beyond “bumping the plug” and define metrics or key performance indicators to quantify cementing success in terms of engineering, operations and economics.
Over the course of this project, the operator and service company developed specific KPIs for defining and measuring success. These same KPIs can be modified and applied to any cementing project to quantify success and look for opportunities to improve efficiencies and reduce costs. This process not only produces better cementing results but also strengthens the working relationship between the parties because both benefit from success.
In the case of this project, successful cementing operations contributed to an overall reduction in drilling and completion costs for the operator and increased cementing market share for the service company.