Well tests and monitoring
Testing well casings
There are a number of tests that operators can use to provide information on the integrity of the well:
- pressure tests are conducted on the casings installed in wells to ensure they have pressure integrity. Leak-off or formation integrity tests are also conducted once the bottom of the casing strings have been drilled out to determine what the strength of the rock is;
- a leak-off test is where the rock is subjected to hydraulic pressure until the drilling fluid begins to leak into the rock and it begins to fracture, this determines the 'leak-off pressure;'
- a formation integrity test is where the rock is subject to a predetermined pressure below the leak off pressure to monitor well integrity. Both of these tests provide the well operator with information on the strength of the rock about to be drilled through, but also provides confirmation that the casing is properly cemented into the section of rock that has just been drilled;
- a cement bond log of casing strings can also be conducted if there is any doubt about the quality of a cementing operation. This determines where the top of cement is in the casing and confirms that the cement is as designed for specific location and of the appropriate quality. The pressures in the spaces between the casings are routinely monitored throughout the life of the well to ensure that integrity is maintained.
All of the above tests represent standard oilfield practice for well construction and are not particular to shale gas operations.
Casings in the well are typically pressure tested as follows:
- once they have been installed and cemented;
- a leak-off test or formation integrity test can be conducted on the rock at the bottom of the casing once it has been drilled out. This is to ensure that the cement bond at the base of the casing is good and to gain information on the strength of the rock at the bottom of the casing;
- the innermost casing will be pressure tested prior to any hydraulic fracturing operations and after the running of any completion tubing. (The completion tubing is casing, through which the gas is produced, and incorporates safety devices to prevent or mitigate escape of gas if the well-head is damaged.)
Cement bond logging and monitoring the integrity of the cement bonding
Cement bond logging (CBL) can be a useful means of verifying integrity where there is a single casing. CBL cannot verify the cement integrity through double casing of pipe and cement and so is one of a number of well integrity tests that will be conducted on the well. Where there is a double casing, the best method and standard industry practice is to monitor the annular pressures. As an additional protection, the EA require surface methane and groundwater monitoring, with any anomalies to be reported to EA, HSE and DECC, and compared with data from the National Baseline Methane Survey, being undertaken by the British Geological Survey.
Ensuring the quality of concrete well casings
The cement specification, testing of the cement and placement of it in the well follows recognised industry best practice as contained in UKOOG Shale gas Guidelines and the following American Petroleum Institute (API) documents:
- API Guidance Document HF1 – Hydraulic Fracturing Operations – Well Construction and Integrity Guidelines.
- API Specification 10A (ISO 10426-1:2009) Specification for Cements and Materials for Well Cementing.
- API Recommended Practice 10B-2 (ISO 10426-2:2003) Testing Well Cements.
Independent monitoring arrangements
Regulation 18 of the Offshore Installations and Wells (Design and Construction, etc.) Regulations 1996 requires the well operator to set up a well examination scheme and appoint a Well Examiner. This is an important quality control mechanism for the industry.
The well examination scheme and involvement of the Well Examiner is for the complete lifecycle of the well from design through to final plugging and decommissioning. The Well Examiner is an independent competent person who reviews the proposed and actual well operations to confirm they meet the well operators policies and procedures, comply with the regulations and follow good industry practice. During assessment and inspection activities, HSE checks that the operator has these arrangements in place.
The well operator's well examination scheme requires the operator to send the following documents to his Well Examiner:
- the well construction programme and any material changes to it;
- reports on how the well is being constructed;
- reports on how the well is being monitored; and
- at the end of the well's life, a plan for how it will be plugged and decommissioned.
The Well Examiner reviews these documents to ensure the complete lifecycle of the well is designed, constructed and operated in line with the Well Operator's policies and procedures, good industry practice and legal requirements.