In heavy oil and other unconventional plays, the majority of new wells producing today are deviated from vertical. Deviated well profiles—which include directional, horizontal and slant well trajectories—place greater operational demands on artificial lift systems. For rod lift, the rod and tubing strings are in close physical contact in deviated wells, creating an abrasive environment that threatens their operating life. In Canada's heavy oil fields, using lift techniques such as progressing cavity pump (PCP) lift to pump highly viscous oil with a substantial sand concentration typically results in significant tubing wear and solids settling at the pump discharge. Conventional coupled sucker rods suffer from pressure loss because of restricted flow at the couplings. Countering this restriction often requires increased pumping power, which subsequently increases lifting costs for the well. At the same time, the presence of sand around each coupling has the potential to cause abrasion, which may lead to premature coupling or tubing failure if left unchecked. One solution is the use of continuous rod, which is essentially one long, continuous sucker rod for the entire length of the wellbore. Also known as coiled rod, continuous rod strings have no couplings, which provides technical advantages. The removal of couplings translates to lower maintenance costs because overall rod stress and tubing wear are reduced, particularly in deviated wells. Surface drives can be downsized, lowering capital and energy costs, and downhole pumps can be upsized, allowing for increased production. The absence of couplings also creates a uniform diameter of the continuous rod, resulting in a reduced pressure losses and a laminar flow profile up the wellbore (see Figure 1).
Continuous rod's uniform diameter allows for in-situ coiled tubing interventions to clean out sand and other fines from the top of the pump. The lack of couplings in a continuous rod reduces the risk of rod fall issues in reciprocating rod lift (RRL) applications and allows for a larger diameter rod to be deployed in slimhole applications. In addition, because a coupling connection makeup is not needed every 25 feet, faster installations are possible with running speeds up to 100 feet per minute. Also, fewer personnel are required for this operation, lowering health, safety and environment (HSE) risks and overall service costs.
A Limited Service Model
With such advantages over conventional sucker rods in deviated wells, the limited use of continuous rod may seem surprising. However, it was not always readily available. Continuous rod debuted into the Canadian RRL market in the early 1970s, and only one service provider supplied it. Many producers recognized the technical merits of continuous rod for their reciprocating pump systems, and the product gained moderate acceptance for use in the deviated, directional and poorly drilled vertical wells of the day. Over time, the product grew in popularity and was made available wherever companies struggled with deviated wells' production issues. By the early 1990s, PCP lift systems were gaining popularity as a lift option for heavy oil wells. PCP systems were particularly suited to pumping viscous oil with high sand concentrations from Canada and South America. The technological advantages of continuous rod allowed it to become the product of choice for many producers using PCPs. As continuous rod sales increased in these basins, keeping pace with the ongoing service requests for intervention work proved difficult. Because patents were held on the product and servicing equipment, additional providers could not enter the market. Despite its field performance success, the reputation of continuous rod was adversely impacted as a result.
Increasing Availability & Service
By the late 1990s, most of the original patents had expired, opening the continuous rod market to other service providers. The technology's popularity in both RRL and PCP applications has increased during the past decade, thanks to broader manufacturing and servicing options, which include:
- An increase in global manufacturing capacity by more than 500 percent in Canada, the U.S. and the Middle East has brought the product closer to the fields that would benefit from its use.
- An exponential increase in the size of the global service fleet has led to a wider, more competitive service contractor base.
- Development of innovative equipment to make continuous rod more adaptive, field welding more effective and transportation methods that allow for more flexibility. The flexibility of the configuration method of the continuous rod injector made it viable for many service companies offer.
Another major concern was the welding component of continuous rod. Every continuous rod string requires a rod end attached to the bottom and top to adapt it to the pumping equipment or the polish rod. Welding is also required for splicing and repair. It was historically a capital-intensive process in the field, mainly because of the large energy source required—multiple deep cycle batteries in series providing a direct current. These systems could not consistently produce quality welds, and their high costs limited their availability. A new, more reliable butt-fusion welding method uses oxy/acetylene to provide a reliable energy source, which equates to consistently better quality welds. The capital cost of these units is one-tenth the cost of previous portable welders. These newer units are also easily transported in the back of a pick-up truck. These benefits have made it possible for many contractors to enter this facet of continuous rod servicing. The producer enjoys a wider range of servicing options and can often afford to bring the welder along with the continuous rod injector to every job.
Proving Its Potential
In 1998, one oilfield technology company entered the continuous rod market. It also developed a continuous rod injector as a rig assist to work in conjunction with most existing well servicing and workover rigs. The first continuous rod injector model was designed for the Canadian heavy oil market. The use of continuous rod lift in this region produced convincing results, with reduced well interventions, decreased service time, improved pump performance, lower energy costs and increased production. As an example, one Canadian operator was experiencing premature rod and tubing failures while using conventional rods to drive PCPs in their 3,500 cold, heavy-oil production with sand wells. The company was producing fluids with high sand concentrations and American Petroleum Institute gravities from 8 to 20 degrees. This contributed to rapid, abrasion-induced wear in the wells and an average run life of nine months before production stoppage for tubing or rod maintenance. The operator needed to decrease well interventions by reducing rod and tubing failures without incurring an increase in operating costs. Continuous rod was identified as a potential solution. The oilfield technology company embarked on a three-year field pilot installing continuous rod in 1,500 of the operating company's new drills. By the end of the three-year run, a total of 19 producer-owned continuous rod injectors were operating in the field. The operator experienced a 33 percent increase in mean time between failures (MTBF), meaning nearly three fewer interventions during the well's life. The time from initial installation to the first intervention, which occurred because of pump failures, increased by 200 percent. The results confirmed that removal of the couplings created several advantages. First, it reduced rod and tubing wear as indicated by the increased MTBF. It established the constant velocity of production fluids up the tubing, which enabled the fluid to carry sand to the surface more efficiently as indicated by the increase in original pump life and MTBF. In wells that experienced sand bridge issues above the pump, operation without couplings allowed for the use of smaller diameter coiled tubing alongside the continuous rod to remove sand down to the pump, preventing a workover rig intervention. Backpressure at the pump was also reduced, which helped extend pump life and increased production by 10 percent through better pump performance. The net well service costs were reduced by $254,000 per well during an eight-year life cycle, 200 percent better than the target. Since the pilot's completion, the operator has adopted continuous rod for all vertical, deviated and horizontal well artificial lift applications. More than 50 continuous rod injectors now service their 5,000 wells. In the heavy oil regions of Western Canada, continuous rod has become the rod of choice in both RRL and PCP lift applications, and this can be largely credited to the accessibility of service equipment.
In 2001, the global fleet of continuous rod injectors was less than 40 units, mainly located in Canada. While the number of installations increased in PCP and RRL applications during the first 25 years of continuous rod's existence, the availability of servicing options allowed the technology to take off. Installations have spread from North America to South America (Venezuela and Colombia) and the Middle East (Oman and Bahrain), with several successful trials completed in Australia and Europe as well. As of 2014, a major artificial lift system provider has placed more than 200 continuous rod injectors into oil fields worldwide, with more injectors continually appearing in the major U.S. basins. This growth is likely to continue as the industry strives to maximize production from mature and unconventional reservoirs through deeper deviated wells with longer laterals.