We need to study the technical and operational aspects of top-hole drilling in the planning phase of a well. This will greatly help us provide practical shallow gas procedures for that particular well. The comprehensiveness of these drilling procedures depends on the chance of encountering shallow gas. The summary of wells with a higher probability of encountering shallow gas is as follows:
- The exploration wells.
- When drilling in shallow gas-prone areas.
- Oil & gas wells identified by a preliminary shallow gas investigation;
- Wells diverter drilled through hydrocarbons (depending on inflow performance).
- When drilling through possible charged shallow sands (leaking or poorly cemented casing pipes);
- Wells with drilling constraints.
Suppose an anomaly indicates the presence of shallow gas. In that case, drill away from this anomaly so that the well will not penetrate any possible shallow gas accumulation.
Pre-Spud Pilot Holes
When drilling offshore wells using bottom-supported offshore rigs with an increased risk of encountering shallow gas, the safest approach is to drill sacrificial pre-spud pilot holes with a soil survey vessel before the drilling rig‘s arrival. Additional benefits are:
- The setting depth of marine conductor and conductor pipe can be optimized;
- If the absence of shallow gas or hydrocarbons is confirmed, there will be no need for stringent shallow gas procedures. This will save rig time and make top-hole drilling procedure more cost-effective.
DIVERTER OR BOP STACK?
The following criteria should be observed when deciding using whether a diverter system or a BOP stack:
- Diverter drilling through formations with an increasing pore pressure regime should be avoided if losses are to be expected below the conductor shoe. The casing scheme should be changed accordingly, and we should consider using a normal BOP stack when drilling through such a zone.
- Many experts recommended avoiding diverter drilling through known hydrocarbon-bearing formations if there is a probability that the well is capable of flowing. The casing scheme should be changed to allow the use of a BOP stack.
Pilot Hole Size In Shallow Gas Drilling Procedure
The probability of encountering a well kick, the chance of dynamically killing the well, and the kick severity determine the pilot hole size in top hole drilling. Small pilot holes will enhance the dynamic well-killing capability the most and improve log quality. Many oil and gas companies generally recommend drilling 9 7/8″ or smaller pilot holes when encountering shallow gas.
Logging, Log Results, and Follow-Up
Further logging will be required if hydrocarbons are suspected or indicated in the pilot hole from the Induction/SP Sonic/Gamma Ray/Caliper log. This will help determine the hydrocarbon-bearing zone’s saturation, porosity, and permeability of the hydrocarbon-bearing zone(s) and estimated flow capacity. Depending on these results, there will be a need to carry out one of the following options:
- Open up the hole and set the casing at the initial casing setting depth.
- Plug back to above hydrocarbons, open up the hole, and set casing (if the resulting changes in casing design are acceptable, otherwise, the well should be abandoned);
- Well abandonment and, furthermore, shifting to a location that may help bypass the shallow hydrocarbon accumulation. If such a location is unavailable, we may consider a less vulnerable rig to a shallow gas kick.
Measurement While Drilling
The electric wireline logs or MWD can inform us about the presence and depth of possible hydrocarbons. The MWD is the preferred method since early detection obviously enhances the safety of the operation. The earliest hydrocarbon detection will be achieved when MWD-logged data is transmitted to the surface whilst drilling. As soon as MWD indicates possible hydrocarbons, we can decide to plug back into the top hydrocarbons zone, enlarge the pilot hole, and set the casing directly above the top hydrocarbons if the resulting changes in casing design are acceptable.
Another advantage of MWD is that the depths of marine conductor/foundation pile setting and conductor string for subsequent wells can be based on lithology rather than an arbitrary figure.
If it is not certain we penetrate the hydrocarbons at the bottom of the hole, the unlogged section of a wireline-logged pilot hole needs to be plugged back. When employing the MWD, and we cannot observe hydrocarbons, plugging back is unnecessary. This makes top-hole drilling more efficient. If hydrocarbons have to be plugged off with cement, it is recommended to have the Top Of the Cement (TOC) not too high above the top hydrocarbons to avoid excessive loss of hydrostatic head when the cement sets.
The gas should be circulated if significant gas readings are obtained from the mud returns or gas bubbles are observed at the seabed when riserless drilling is employed. If the background gas level cannot be reduced by circulation, increase the MWT. In addition, you can circulate the hole until the background gas subsides. The pilot hole should be logged at the first opportunity,. After that, appropriate action should be taken depending on the log results.
In development drilling, where there is sufficient geological information available from surrounding wells to determine that there is no shallow gas, logging may not be required, which avoids pilot hole drilling and hole opening operations. However, the use of a diverter system is still recommended if there is a chance of colliding with another well or if there is a possibility of penetrating charged sands from leaking or poorly cemented casing strings.
Losses While Shallow Gas Drilling Procedure
Mud losses should be avoided during drilling with a diverter system installed. If any losses are encountered, they can be remedied by using lost circulation material (LCM) or cement. Full returns are to be regained before proceeding to drill ahead. If losses cannot be remedied, possible solutions include cement plugging, casing placement adjustment, or well abandonment.
Drilling with total losses and using a mud cap may be considered where the geophysical structure is well known. In addition, we can use it when total losses are a common problem, and where this technique has been proven feasible.
Shallow Gas Kick Whilst Tripping in Drilling Procedure
If a kick is experienced whilst tripping, the first option should be to pump all available kill mud as soon as possible to kill the well dynamically. If the well is successful and the flow stops, return the string to BTM to circulate the well.
Depending on flow severity, when the flow fails to stop with kill mud, strip the string back to the BTM.