Surface casing vent flows and gas migration are becoming increasingly difficult and expensive to repair. By following a process, which includes; understanding gas migration mechanisms, well history review, area information and some inexpensive testing before repairs begin, source detection can become easier and more accurate. This same information can then be used for post job evaluation. Suggestions for cased hole logging evaluation, perforating and squeeze techniques are given to reduce the chance that poor job design will be the cause of repair failure.
A seven step process, as shown in Figure 1, has been developed as a systematic approach to gas migration source determination and repair. The term gas migration will be used throughout the paper to refer to both surface casing vent flows and soil gas migration. By following the process it is possible to reduce the cost of source determination significantly, by reducing costly logging and other downhole investigations.
The process increases the success of repairs on leaking wells by more accurately identifying source intervals and improving designs for repair operations. The process allows for evaluation of repairs to capture leanings. These learnings can be applied to other wells or subsequent repairs if required in the same wellbore.
Documentation of information gained from following an iterative process of improvement will increase success rates in a particular area as increased knowledge is gained.
For gas migration to exist there are two requirements:
1. There must be a source formation where gas or liquid hydrocarbons exist in the pore space under some drive pressure.
2. There must be a conduit for gas to flow to surface. The following mechanisms of gas migration describe the creation of the conduit to surface that allows gas to migrate to surface.
In open hole abandonments cement plugs are placed in the open hole to seal the wellbore. This seal is intended to stop zone to zone flow within the wellbore and flow to surface. Proper plug placement in the open hole is imperative to stopping gas migration in drilled and abandoned wellbores.
Plug effectiveness is dependent on many things such as proper interval selection. Plugs should be placed across potential source intervals, not above them. If plugs are placed above the zone there will be a reduction of hydrostatic head which may lead to gas migration through the cement plug.1 Consideration must be given to the wellbore fluid density that the plug is to be set in, large density differences between cement and wellbore fluid can allow the plug to fall prior to setting. Factors such as wellbore size, deviation and open hole condition must be considered and designed for when placing balanced plugs in open hole. 2Poor cement plug placement can lead to very costly re-entries for gas migration repair.
Gas migration through cement occurs when the hydrostatic pressure is reduced during the transition period.