THE WELL BORE

When a drilling rig is positioned, whether it is a land rig or offshore vessel, the drilling operation is ready to commence.

Typically, a wide conductor pipe, up to 36 inch in diameter, is forcibly driven into the surface sediments by repeated hammer blows. The sediments can then be drilled out from the inside of the conductor pipe with returns and cuttings circulated via a diverter.

Driving the pipe, rather than drilling a hole first, prevents the surface sediments from being washed out and weakening the foundations of the rig. A firm anchor is therefore provided for the installation of the BOPs. On jack-up rigs, this provides an immediate link between the wellbore and the rig and BOP stack.

Alternatively, the hole may be drilled first before running conductor pipe. When the surface formation is first penetrated by the bit, the well is said to have been spudded. The hole may be drilled in one go with a large bit or it may be drilled first with a smaller bit and then re-drilled with a larger diameter hole opener.

Offshore floating rigs drill this first hole section open, allowing the seawater to act as the drilling fluid and return the drilled cuttings to the seabed.

Before drilling can go any further the hole must be sealed off to provide a closed system. This allows a drilling fluid to be continually recycled and drilled cuttings collected and examined. A wide diameter pipe, equivalent to the conductor pipe but now called casing, is run into and down to the bottom of the drilled hole.

A cement mixture is then pumped into the casing and forcibly displaced so that it fills the space between the casing and the formation. When this cement has set, the well is sealed so that when drilling recommences, the drilling fluid as well as any formation fluid is safely returned to the surface via the inside of the casing.

Again, when set, this casing prevents any collapse of the surface sediments, which may typically be weak and unconsolidated, providing a firm foundation and a firm anchor on which to position the BOPs.

In general, the BOP stack is installed when the casing is set, although in some cases, operators wait until the surface hole is drilled and casing set. In the case of jack-up rigs and land rigs, the BOPs are installed directly beneath the rig floor. A flow line is then connected to return drilling mud and cuttings to the surface circulation system.

In the case of offshore floating rigs, the BOP stack is installed on the seabed where the casing strings terminate. A marine riser, which includes a telescopic or slip joint to allow for vertical movement of the rig because of tidal and heave motion, links the BOP stack to the rig completing the closed system. A diverter

is always installed as part of the surface flowline system, so that, if the well cannot be controlled by the BOPs, and returns are reaching surface, gas can be directed safely away from the rig.

5.1.2 Surface Hole

This hole section is drilled to a determined depth and again sealed off by running casing to the bottom of the hole and cementing it in place. The base of the casing, or shoe, is generally the weakest part of the next hole section simply because it is the shallowest point and subject to the least overburden and compaction.

The depth and lithology to which the surface hole is drilled and the casing set is therefore critical. The lithology where casing is set should be consolidated, homogeneous and with low permeability. The competence of this lithology must provide sufficient fracture strength to drill the next hole section with a sufficient safety margin over any formation pressures expected (see Section 5.6 Pressure Tests).

The surface hole is of wide diameter and normally drills quite rapidly because the surface sediments are not too compact or consolidated. A large volume of cuttings is therefore continually produced. To ensure these cuttings are removed from the annulus and so to prevent them blocking or impeding the movement and rotation of drillstring and bit, viscous mud sweeps are made at regular intervals. This simply involves a volume of thick, viscous drilling mud being circulated around the entire hole. The viscosity of the mud enables it to lift and carry all of the cuttings out of the hole.

The surface hole is normally completed with just one drill bit. If the bit should wear out however, it is replaced by lifting the entire drillstring out of the hole (tripping, see Section 5.4). This is done by breaking the drillpipe into lengths of 3 (triple stand) or 2 (double stand) joints, depending on the size of the derrick.

When the hole section is completed and before the casing string is set in place, the operator normally requires the hole to be logged with electrical tools to gain specific information about the wellbore and lithology. These tools are run into the hole on a thin wire and are therefore termed wireline tools.

The wireline tools are very expensive but the wire can only be subjected to a certain amount of load before it would snap. Therefore, before logging, a wiper trip is performed. This operation ensures the hole is clean and not closing in at any point. It involves raising the drillstring part way out of the hole or until the bit is out of the open hole and inside the previous casing. The bit is then run back into the bottom to determine the condition of the hole. If any tight spots are encountered, they can often be corrected simply by working the pipe up and down through the tight section; circulating at the same time helps the cleaning.

If the hole is really tight or undergauge, it may seriously restrict the movement of the pipe or even not allow the bit to pass at all. In this situation, the tight section will need to be effectively re-drilled or reamed with full circulation and rotation.

When the bit reaches the bottom of the hole, a bottoms-up circulation is performed. This ensures that any cuttings that may have fallen, or have been dislodged during the hole cleaning, to the bottom of the hole

are lifted and circulated out of the hole. This enables the logging tools to be run all the way to the bottom of the hole.

Once the hole section is logged, casing can be run and cemented in position. The main purposes of the surface casing are:

 Provide a firm and competent anchor for the BOP equipment.

 Protect formations from further erosion.

 Seal off fresh water aquifers from any contamination.

 Prevent collapse of unconsolidated formations.

 Seal off any subnormal or over-pressured formations.

Before drilling ahead with the next hole section, the BOP stack and casing are pressure tested to ensure there is full integrity and that all prevention equipment is fully functional.

5.1.3 Intermediate Hole

Before this section can be started, rubber plugs and cement remaining from the cementing of the previous casing must be drilled out before new lithology is encountered. Just a small interval of the next hole section is then drilled, typically 5-to-10 meters, and then a pressure test performed.

This Leak Off Test (LOT) or Formation Integrity Test (FIT) determines the integrity of the cement bond and also allows determination of the fracture pressure of the formation at the shoe. The fracture pressure is the maximum pressure that can be exerted on the wellbore without fracturing that formation, a situation that must be avoided at all costs.

Exactly the same procedures are followed as in the surface hole, such as drilling, tripping, logging, casing and cementing.

The exact number of hole sections a well has is dependent on several factors, such as:

 Depth, fracture pressure and kick tolerance of the previous casing shoe.

 Hole / formation problems that may be encountered such as zones of lost circulation, unstable formations, abnormal formation pressures, pipe sticking problems.

 Change of mud type to a system that may be unsuitable or damaging to particular formations

All of these situations may result in an intermediate string of casing being set to seal off a particular interval. Each subsequent casing string is run from the surface, inside the previous casing, to the bottom of the hole. This new string may be cemented all the way back to surface, but it is normal to cement it back to the inside of the previous casing which is cemented back to surface.

5.1.4 Total Depth

As the total depth (TD) of the well is approached, any casing that may need to be run is normally run into the hole on drillpipe and hung from a hanger inside of the previous casing. In this situation, it is termed a liner, but procedures for cementing and testing are exactly the same as for any casing string. Obviously, as the well becomes deeper, the casing requirements become much more expensive if it were to be run all the way back to surface.

Situations vary, but the well may be drilled through a prospective production zone to the well’s TD, or it may be drilled to just above the production zone and the liner set in place. This situation would enable any problem zones previously encountered to be sealed off and the production zone isolated; it would allow the mud system to be changed or modified specifically for the zone of interest in terms of formation and production protection and pressures expected.

Depending on operator requirements and on indications when drilling into the zone of interest, for example, rapid drilling to indicate porosity, gas or oil shows from the drilling fluid, the interval may or may not be cored.

Cutting and preserving a core of the reservoir interval allows much more precise laboratory analysis to be carried out regarding the productivity and economical potential of the reservoir. Cutting a core requires the use of a specialized core bit that cuts around and leaves a central core of rock, typically around 10cm in diameter, intact.

As the bit cuts down and deepens the well, this core moves up into a special sleeve and core barrel that holds the core. At the end of the coring operation, the core is held in the barrel and it must break off from the bottom of the wellbore by physically lifting the string until the core snaps off. This is a very important operation to ensure the core is retained and does not fall out from the barrel (See Section 5.3.1).

At TD, wireline tools are run. A fuller array of evaluating tools may be run if the zone of interest shows good hydrocarbon potential. If a core hasn’t been cut, sidewall cores may be cut with a wireline tool from specific depths of interest.

If the zone shows producing potential, the well may be production tested with a drillstem test (DST). A production casing string is run to the bottom of the hole and cemented in place. This casing can then be perforated at specific depth intervals that correspond to the zone of interest (See Section 13.9).

The production casing will be displaced to a specialized fluid or brine, the density of which allows formation fluids, including oil and gas, to flow into the wellbore. Testing equipment, known as a Christmas tree is installed at the surface to measure and determine the reservoir pressure and flow rates.

Figure 38: Christmas Tree

When all work is completed, the well is plugged with cement to isolate any open hole or production zones from the surface. If there is no reservoir potential, the well is abandoned; if there is potential the well is suspended to allow for further analysis and testing to be completed.