evaluating connection challenges - C-FER Technologies [PDF]

companies, equipment manufacturers, and secondary stakeholders. (consultants, etc.). Historically, some of the resulting

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Idea Transcript


EVALUATING CONNECTION CHALLENGES KIRK HAMILTON, C-FER TECHNOLOGIES, REVIEWS THE EVOLUTIONARY CHALLENGES OF TODAY’S OILFIELD FROM AN OCTG CONNECTION EVALUATION PERSPECTIVE.

T

he pace of change in the oil and gas industry is accelerating and sometimes it is difficult for technology providers to keep up. The industry still refers to ‘unconventional’ tight oil and gas reservoirs, even though they are now common and are the primary assets for many operators, especially in North America. One of the key technologies driving this change is multi-fractured horizontal wells (MFHW). Five years ago a complex MFHW may have consisted of up to five or six fracture intervals; however, now operators are developing wells with ten times this number of fractures in extended reach wells. Furthermore, sometimes as wells age, the rate of production declines faster than predicted and operators need to refracture these wells to restore economical production rates. The induced low-cycle fatigue loading of the repeated pressure and temperature cycles of repeated hydraulic fracturing operations are pushing the service limits of both surface and

downhole equipment, and require a new approach when it comes to assessing equipment and tubular suitability for MFHW applications. Industry stakeholders are working hard to ensure that well design and equipment manufacturing standards keep pace with these changes, in order to ensure that these wells can be operated in a safe and environmentally responsible manner.

THE IMPORTANCE OF RECOMMENDED PRACTICES Equipment and oil country tubular goods (OCTG) used in wells with extreme loading conditions, such as elevated temperature, sour environment, and high pressure, require comprehensive performance evaluation and validation prior to being put into service. The American Petroleum Institute (API) has long been recognised as the gold standard with respect to developing recommended practices and standards for equipment and material performance/conformance assessment for the energy industry. These recommended practices and standards are the result of extensive work performed by committees of volunteers from operating companies, equipment manufacturers, and secondary stakeholders (consultants, etc.). Historically, some of the resulting API documents have been sanctioned by the International Standards Organization (ISO) to form the basis for global references (e.g. API TR 5C3 1st Ed./ ISO TR 10400:2007) and standards (e.g. API RP 5C5 3rd Edition/ISO 13679:2002). These efforts are vital to the industry to ensure a level of self-governance by demonstrating to regulators that performance assessment and evaluation processes are rigorous and fit for purpose. Evaluation protocol standardisation and the use of industry

Figure 1. API RP 5C5 Series B TLE sealability test on a 3.5 in.

diameter OCTG premium connection specimen.

| Reprinted from Oilfield Technology

May 2017

recommended practices enables end users to evaluate different designs and/or materials using consistent methodologies that have been vetted by industry experts to select appropriate equipment for their specific applications. The performance-based nature of these assessments allows well designers to adopt new technologies to meet the evolving needs of their operations. To be fit for purpose in a changing industry, recommended practices and protocols must be continuously updated. A good example of this is the recently published RP 5C5 (4th Edition) ‘Procedures for Testing Casing and Tubing Connections’ which expands upon previous versions of the recommended practice developed to assess the performance of OCTG and premium connections by addressing complex in well loading environments such as extreme the external pressures and temperatures seen in offshore HPHT applications.

UNDERSTANDING RECOMMENDED PRACTICES Equipment evaluation protocols described in recommended practices can be very complex. In the case of OCTG premium connection evaluation, some protocols such as RP 5C5 require rigorous material characterisation testing and full-scale testing of multiple connection specimens of various ‘worst case’ geometric tolerance combinations to assess the effects of variability in the manufacturing process. Each premium connection test specimen selected for evaluation under RP 5C5 is subjected to a battery of load sequences, often at extreme loads approaching the limits of material and connection strength, which are selected to represent the anticipated application environment. Furthermore, connection specimens are required to meet or exceed specified performance thresholds. In particular, connections must demonstrate their sealability performance under a wide range of combined loading scenarios by exhibiting fluid seepage rates of less than 0.9 ml during each 15 minute interval of a hold at a specific load point in the test matrix. While this may seem like a significant value, it translates to a rolling average rate of only 0.06 ml/minute. Additionally, the trend of connection seepage during each hold at a load point must also be characterised as declining, stable, or increasing to further assess the long-term performance of that connection design. Failure to meet these thresholds indicates that the connection may not be suitable for the intended application. The fact that the outcome of an RP 5C5 test programme has the potential to dictate the suitability of a premium connection design for a specific application means that it is very important that the party performing the evaluation (i.e. the test lab) does so with a high degree of technical expertise, quality, and transparency. While the recommended practice describes testing requirements (the ‘what’), it does not contain vital expertise in test execution (the ‘how’) to achieve high quality, representative and fair test outcomes. Examples of vital expertise include documented and fit-for-purpose procedures for tasks such as applying strain gauges to test specimens, and ensuring data quality and repeatability through redundant parameter monitoring (e.g. multiple thermocouples at various locations on the specimen to determine temperature distribution rather than a single point measurement). Automation of test load control systems, such as pressure metering for external and internal pressure control, pre-programmed heater ramp rates, and computer-controlled axial

loading, reduce the likelihood of overshooting load targets during testing and potentially damaging test specimens already being pushed to their design limits. To assist in ensuring the transparency of testing programmes, test protocols like RP 5C5 often mandate the use of third-party inspectors to witness testing on behalf of the end user and/or connection manufacturer. Inspectors will take the time to go through and examine all aspects of the test from the setup to assessing the sensitivity of connection sealability measurement systems. This should also include reviewing calibration certificates for instrumentation being used on the test programme before the test starts to ensure that the data collected is accurate.

INDUSTRY GROWING PAINS Given the universal acceptance of RP 5C5 as the global reference for connection evaluation, MFHW operators have assessed connection performance using either the most stringent test under RP 5C5 (Connection Application Level IV), or some combination of loading conditions specified in the RP 5C5 test load envelope for assessing sealability. Some connection manufacturers and operators have created custom evaluation protocols for their applications, but these have not been universally accepted. In 2016, various industry representatives were pointing out that there was a need for a separate connection evaluation protocol to address the loads specific to MFHW because, as MFHW designs have evolved, questions have been raised as to the suitability of RP 5C5 when it comes to evaluating connection performance for MFHW for a variety of reasons. First, although RP 5C5 was designed to be applicable to as many industry sectors as possible, it is mostly used to assess the performance of premium connections for high-pressure, high-temperature (HPHT) applications, in particular offshore applications wherein tubular performance is particularly critical (a connection failure could result in a serious well control incident). The loads that connections are evaluated to under RP 5C5 are reservoir/depth related and are for the most part static loads, whereas the critical loads of MFHW applications are generally driven by completion and production practices, which are often specific to each operator’s well or field. The key loads that are unique to MFHW (i.e. not considered in RP 5C5) that have the potential to cause low-cycle fatigue failures of the OCTG connections are:

COMBINED ROTATION AND BENDING This load condition is very common in extended reach MFHW and occurs when casing is run through a build section to get down to the horizontal/deviated production zone. In these situations, crews running tubulars through the build section of the well will often rotate the casing to reduce drag and friction. Rotating tubulars, while effective at achieving running depths, can induce extreme loads on connections, especially in the threads. Moreover, in addition to the combined rotating and bending, there are the induced drag load effects of increased torque and axial load that also need to be considered with respect to fatigue loading.

HIGH PRESSURE RATE AND MAGNITUDE CYCLING This load condition is present during the hydraulic fracturing process. Tubulars are exposed to rapid changes in internal pressure as a result

| Reprinted from Oilfield Technology

May 2017

of this completion practice, and now that operators are performing up to 50 fracturing cycles prior to production, this pressure cycling may have a significant impact on tubular performance, especially in materials that are susceptible to the effects of near-yield cycling (e.g. high-chrome alloys).

THERMAL SHOCKING This load condition needs to be considered for wells in colder regions that are being produced from elevated temperature production zones. Field data shows that fracturing fluids injected at low temperature can be considerably warmer when they return to surface, suggesting that a heat exchange process occurs downhole. The impact of introducing cold fracture fluids to a hot wellbore can result in sudden material property changes that may lead to reduced performance (i.e. sealability) or structural capacity. Second, although not stated in the title of RP 5C5, the intended candidate connections evaluated under the RP are virtually always premium connections, either integral or threaded and coupled. Many MFHW wells use semi-premium or even API connection designs throughout the well. These types of connections are not designed to withstand the internal and external pressures that RP 5C5 subjects test specimens to. Furthermore, the stringent sealability criteria established by RP 5C5 (0.9 mL/15 minutes) may not be as critical for many MFHW applications as compared to offshore HPHT applications. It is likely that some other sealability criteria will be established for MFHW that will be less demanding than RP 5C5 in this regard. Recognising the shortcomings of RP 5C5 for MFHW connection evaluation, a new committee was formed by the API, charged with the development of a protocol specific to MFHW. The underlying objective of this committee is to create a protocol that can adapt quickly to the evolving nature of the MFHW sector. This will be achieved through enabling customisation, flexibility, and expansion of the test programme (to account for changes and evolution of the MFHW practices) to suit industry needs while maintaining a framework of necessary evaluation criteria. This approach will enable operators to compare performance between different connection designs. However, this protocol is not expected to be published until 2019, so operators keen to validate connections for use in these applications will need to continue with custom, fit-for-purpose evaluation approaches for the time being.

THE HORIZON As the example of MFHW shows, the industry must be mindful that equipment evaluation protocol development must keep pace with the ever-evolving requirements of new applications and new operating procedures that may impose new loading conditions or performance requirements on equipment. Adopting a flexible, adaptable framework for these protocols will allow operators to customise the evaluation programmes to suit their specific needs. Regardless of the effort to develop more rigorous evaluation protocols, operators will still be required to interpret the results of these evaluations to determine if the equipment is suitable for their application.

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