Geophysical Investigations of Cased Well Completions

Abstract

AbstractMonitoring and production wells must be carefully completed to ensure protection of ground water resources. public health. and quality of hydrogeologic data. Poor completions and annular leakage can cause ground water to be lost to other aquifers, discharged to the ground surface, or contaminated by surface water or other ground water. Knowledge of and confidence in specific well completions are vital to the interpretation of potential well problems and hydrologic data.Once a well is finished, the only access for inspection is via the casing. Water well completion forms provide only a record of the intended completion and offer few insights into completion (or post‐completion) problems. Borehole geophysical tools, however. can provide valuable completion data in certain settings.A variety of geophysical equipment and techniques is available to evaluate cased wells. The availability and cost of some oil‐field techniques, however, are sufficiently prohibitive to discourage use in water wells and monitoring wells. Standard, readily available borehole geophysical tools can provide useful information to verify or refute a completion report. To apply borehole techniques to completed wells, however, the tools must be calibrated by logging known completions in a controlled setting.This project included laboratory calibration tests of omni‐directional and focused gamma‐gamma density tools, and field tests involving these logs plus caliper, natural gamma, neutron, electrical. and sonic logs. Tool responses were measured in 312 laboratory tests representing different tools and well completions; the field tests were conducted in nine monitoring wells in eastern Montana.The study demonstrated a suite of readily available, cost‐effective tools that could identify most completion components in actual monitoring wells. Caliper. (4Pi) gamma‐gamma density, and full‐waveform sonic logs are probably the most valuable of the tools tested. Casing material was clearly distinguished by all nuclear tools. Grouts were identified using omni‐directional 4Pi density tools: in some situations, focused gamma‐gamma density and neutron logs were also helpful, Grout thickness was difficult to determine in the. field settings, to Each individual tool should be calibrated in known settings prior to use in field investigations, and the results carefully evaluated.