Ultrasonic Method for Evaluation of Annular Seals for Wells and Instrument Holes

An ultrasonic testing method employing the pulse-echo inspection technique was developed to assess the integrity of annular seals surrounding casings (i.e.. instrument tubes or well casings). This nondestructive testing method permits testing aseal from inside acasing without disturbing the casing, seal. or formation. Seals constructed with cement-based and bentonite-based sealants surrounding various Types and sizes of casings can be evaluated using the method. An evaluation is conducted by sending and receiving ultrasonic waves using a single piezoelectric transducer and commercially available hardware (a pulser receiver and a waveform analyur). A probe was designed and constructed for downhole testing. Differences in The ultrasonic responses of the malerials in contact with a casing are: analyzed to determine the presence of different materials (seal or defects filled with air or water in a seal) outside a casing, Drilling boreholes is common practice in many disciplines rang· ing ftom geotechnical investigations to mine exploration, Often a pipe is inserted into a drilled hole such as inclinometer or piezometer tubes (in instrument holes) and well casings (in water supply or monitoring wells). These pipes will be referred to as "casings" in this paper. When a casing is placed in a borehole, an annular space is created between the casing and tile surrounding soil. If not properly sealed, this annular space can be a potential path for transport of contaminants in the subsurface environment Cross contamination due to commingling of clean and contaminated groundwater can occur. A poor annular seal can also result in the loss of groundwater. Thus, proper installation and management of casings is necessary to protect tile subsurface environment. Furthermore, a properly placed seal also protects The casing against corrosion and chemical degradation (Nielsen and Schalla 1991; Landry 1992). In this paper, a downhole ultrasonic method is described that can be used to assess the nature of materials (seal or defects filled with air or water) in contact with casings placed in boreholes. The ulrrasonic evaluation is a simple, yet sensitive testing method to assess seals without disturbing the casing, seal, or formation. The method can be used in casings made of metal or plastic. Seals constructed with cemenl and bentonite can be tested. Existing Seal Evaluation Methods The most common methods used for in situ evaluation of seals around casings are: water level monitoring, pressure testing, and cement logging (Driscoll 1986). These methods are used primarily for water supply or oil wells having large-diameter casings. They are less frequently used for instrument holes or monitoring wells having small·diameter casings. A summary of existing in situ seal evaluation methods and their advantages and disadvantages is presented in Table I. Level monitoring involves moniloring changes in the level of water or drilling fluid in the casing. If the seal is intact, virtually no change in liquid level is expected. This method requires simple instrumentation and can be conducted repeatedly after seal placement to monilor the performance of the seal in time. However, the fluid level in the casing is a crude indication of the quality of the seal. The fluid level is not affected unless the defects in the seal are hydraulically connected to the bottom of the casing or to leaky connections. Also, arches formed in the seal cannot be detected and, if poor performance is detected, the location of the defect cannot be identified. Leaks at the bottom or top of the casing can also be misinterpreted as a defective seal. Pressure testing is used where the subsurface is primarily rock. This method can be used only 10 evaluate cement seals. The casing is pressurized under a pressure of 69 kPa for at least 1 h after the cement has cured. If the cement seal is intact, virtually no drop in pressure due to leaks occurs over time. Pressurizing the casing can be conducted any time after curing of a cemenl seal, which allows monitoring of the seal over time. However, when soft formations exist around tile casing, pressure changes do not necessarily reflect the integrity of a seal. The surrounding formations can be compressed, and tile pressure drop can be misinterpreted as a defective seal. Cement logging is a nondestructive testing method that has been used in tile oil and gas industries. Cement logs are designed to evaluate tile inTegrity of cement seals around steel oil and gas pipes using sound waves (Schlumberger 1981, 1989). Recenl appli· cations include evaluation of cement seals around waste disposal and deep water wells (Driscoll 1986; Landry 1992). Most cement logs supply information about the integrity of a seal, as well asinfonnation regarding the casing and the subsulface. Several cement logging tools arc available that fit into different diameter casings. Location of defects between a seal and casing, between a seal and formation, and location of channels in a seal can be determined. In addition, cement logging can be conducted in a casing at different times to monitor the seal over time. The main disadvantage of cement logging is the high cost associated with testing. Logging services are provided by a limited number of companies using patented equipment and specially trained personnel. Data acquisition and analysis aR complicated. In addition, only steel casings and cement-based seals are evaluated with these tools. Two other less frequently used methods for evaluating cement seals are temperature logging and radioactive logging (Driscoll 1986). Temperature logs are conducted within 12 to 24 h after seal placement. The casing is filled with water, and the heat produced during curing of the cement is monitored by measuring the temperature of water inside the casing. The amount of heat produced is compared to that generated by a cenain mass of cement in a laboratory or field model. A lower temperature in the casing than the pre-determined value from the model is indicative of a defective seal. In radioactive logging, a radioactive tracer is mixed into the cement sealant prior to placement. Radioactivity is monitored to verify the position of the cemenl after the seal is placed in the annular space. This method is expensive and requires special procedures for handling of radioactive material. In addition, the radioactive material affects the natural radioactivity of earth. This can result in misinterpretations of nuclear geophysical studies conducted in the vicinity of the casing. A variety of materials are used for seals (bentonite. cement. etc.) and casings (steel, PVC. etc.) in geotechnical and geoenvironmental applications (Lutenegger and DeGroot 1994). On the basis of the limitations of the existing methods, there is need for a simple, yet sensitive testing method to evaluate this wide range of commonly used materials. This method should also allow for repetitive tests after seal placement to monitor the performance of a seal over time. For this reason, a nondestructive testing method that uses ultrasonic principles was developed.