Drilling Engineering and Operations

                        Drilling Engineering and Operations

 Following the preparation stage of field development i.e., setting the produc­tion strategy, determining the locations of the wells in the field, and design­ing the well completions, the drilling-related activities begin. The drilling program is first designed.

 Then, plans are prepared and executed to acquire the required equipment and materials. The drilling sites in the field are then prepared for the equipment and materials to be moved in, and the drilling operations begin. Depending on the organization of activities within the oil company, drilling engineers may only be responsible for drilling and cas­ing of the well, and production engineers will be responsible for completion of the well.

Alternatively, drilling engineers may be responsible for drilling and completion of the wells. The drilling program consists of three main stages: (1) drilling the hole to the target depth, (2) setting the various casings, and (3) cementing the casing.

Whether onshore or offshore drilling is carried out, the basic drilling system employed in both the cases will be the rotary rig. The parts of such a unit and the three basic functions carried out during rotary drilling operations are as follows:

Torque is transmitted from a power source at the surface through a drill string to the drill bit. A drilling fluid is pumped from a storage unit down the drill string and up through the annulus.

 This fluid will bring the cuttings created by the bit action to the surface, hence clean the hole, cool the bit and lubricate the drill string.

The subsurface pressures above and within the hydrocarbon-bearing strata are controlled by the weight of the drilling fluid and by large seal assemblies at the surface (BOPs).

However, in practice, onshore and offshore drilling units are often quite different in terms of technology and degree of automatization.

This is largely driven by rig availability, costs and safety considerations. We will now consider the rotary rig in operation, visiting all elements of the system.

The type of rig operation described first is now found mainly in low-cost onshore areas.

 For complicated, more expensive wells, older rigs have usually been upgraded to include a top drive system and automated pipe handling. New rigs are usually built with this equipment as standard.

When drilling through normally pressured formations, the mud weight in the well is usually controlled to maintain a pressure greater than the formation pressure to prevent the influx of formation fluid.

 A typical overbalance would be in the order of 200 psi.  A larger overbalance would encourage excessive loss of mud into the formation, which is both costly, and may damage the reservoir properties.

If an influx of formation fluid into the borehole did occur due to insufficient overbalance, the lighter formation fluid would reduce the pressure of the mud column, thus, encouraging further influx, and an unstable situation would occur, possibly leading to a blowout. Hence, it is important to avoid the influx of formation fluid by using the correct mud weight in the borehole.

When drilling through a shale into an over pressured formation, the mud weight must be increased to prevent influx. If this increased mud weight would cause large losses in shallower, normally pressured formations, it is necessary to isolate the normally pressured formation behind casing before drilling into the over pressured formation.

The prediction of overpressures is therefore important in well design. Similarly, when drilling into an under pressured formation, the mud weight must be reduced to avoid excessive losses into the formation.

If the rate of loss is greater than the rate at which mud can be made up, then the level of fluid in the wellbore will drop and there is a risk of influx from the normally pressured overlying formations.

Problems can be encountered in the drilling of any hole at any time; and drilling personnel must be aware of this fact and of the symptoms that indicate the various types of problems.

 On the other hand, many problems can be avoided or minimized by proper planning. Geological information and the experience gained from drilling previous wells in the same area can be used to predict the existence of problem zones, and such data should certainly be used to the fullest extent.

However, problems encountered in one well may not exist in the immediate vicinity, while new problems can arise at any time.  Such is the nature of drilling a small hole to great depths through often heterogeneous layers of subsurface rock.

After the shale site construction, the drilling rig is moved on site and assembled. A conductor hole is predrilled, and then conductor pipes are inserted to prevent soft rocks from caving and conduct drilling mud from bottom to the surface during drilling process.

 Depending on the number, depth, and length of horizontal wells to be drilled,  the drilling stage can last for a few months, which requires a constant supply of drilling fluid and proper handling of sediments and wastewater.

Once the drilling is completed, protective casing and cementing are used. The following stage is the well completion, which mainly involves the hydraulic fracturing operation.

 A mixture of water, sand, and chemical additives is injected underground at a high pressure to break up shale-rock formations, such that fractures are created and held open by proppant, and then shale gas and oil can be extracted.

 Typically, the horizontal wells are stimulated by stages, depending the specific fracturing schedule and technology applied, the hydraulic fracturing stage could last for several months.

Once fracturing is completed, a wellhead is constructed, and the local gathering pipelines are prepared for the controlled extraction of natural gas.

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