2.4.1 Directional drilling, horizontal drilling, multidrains
The principles underlying development drilling are the same as those for exploration drilling, but more specific use is made of directional and horizontal drilling, and multidrain systems.
Modern drilling can be controlled so accurately that wells can be drilled according to a precisely predetermined profile so as to target a precise subsurface location.
Directional drilling can be carried out in a J or an S configuration. It is normally used:
– When the drilling zone is inaccessible or urbanised;
– To circumvent a subterranean obstacle such as a salt dome;
– To reduce the number of surface drilling installations, for example to limit the number of platforms when drilling offshore, or to obviate the need to move them;
– To test several potential reservoirs;
– To deal with a well in which there has been an accident.
Horizontal drilling is a special case of directional drilling in which the borehole is hori-zontal, parallel to the reservoir strata. As indicated in Fig. 2.20, it is used:
– When the production zone is a long way from the drilling rig; this technique can even be used to access resources under the sea bed from an onshore location, thus avoiding the need for offshore equipment;
– To enhance productivity, and therefore recovery; by draining a reservoir over a length of, sometimes, more than a kilometre, the oil flow rate can be increased, making it feasible to develop an oilfield of small thickness or low permeability;
– To prevent the local deformation of the oil-water or gas-oil contact close to a producing well (known as coning) which occurs with traditional drilling, which results in an excessive production of gas and water.
Multidrain wellsallow production from different parts of a reservoir with a single well.
They can be used at any stage in the life of a field.
In the exploration and appraisal stages, sidetracking provides a less risky and lower cost means of delineating a field in unknown areas. The profitability of a production well is assured by the main wellbore drilled into a known reservoir (Fig. 2.21).
Chapter 2Oil and gas exploration and production
Sidetracking Offshore Drilling from
the coast
Emergency operations
Multiple zones Inaccessible
site
Figure 2.20 Horizontal and directional drilling.
During production, multidrain systems multiply the number of wellbores and therefore increase production while reducing the development costs per barrel. Drilling multidrain systems in existing wells in the depletion phase slows the rundown of mature fields by tapping into secondary reservoirs and allows a programme of water or gas injection to be carried out for optimal flushing of producing formations.
2.4.2 Completion
Completion involves making the well ready for production. It begins when the drilling phase comes to an end, when the last piece of casing has been cemented into place in the producing formation. First of all a connection has to be made between the wellbore and the reservoir, by drilling into the reservoir, treating it, equipping the well and putting it into production.
The equipment and methods used in well completion are quite varied, depending on the type of effluent, the kind of reservoir, the requirements to be met by the well during its lifetime and the economic circumstances at the time of drilling. The completion must at least ensure the integrity of the walls of the hole and the selectivity of the fluid or production level while permitting the unhampered flow of the fluid. It must ensure that the well is secure, allow measurements to be made, facilitate maintenance, allow the flow rate to be regulated and the well to be put back into production.
Wellbore-reservoir connection
There are two types of wellbore-reservoir connection: cased hole completion and open hole completion.
Cased hole completion is the most common. After the reservoir formation has been drilled the last piece of casing or liner is set and cemented in place. Perforations are then made at the level of the production zone to reestablish a connection between the reservoir and the well. These perforations must pass through the casing and the cement sheathing before penetrating the formation, which may then be subjected to stimulation treatments.
Chapter 2Oil and gas exploration and production
Figure 2.21 Multidrain wells.
In open-hole completion the well is simply drilled into the reservoir, which produces in an open hole. A variant of this involves placing a pre-perforated liner against the wall of the formation, so as to maintain its general shape. This type of completion tends to be used when there is a single zone only which is either highly consolidated or where sand control by gravel packing is adopted. In practice this procedure is rare for oil wells, but is sometimes applied on gas wells.
Tubing
The configuration of the tubing mainly depends on the number of production levels and the production selectivity sought.
In conventional completion, we generally use a tubing which is totally contained in the casing string. Completion may be single or multiple. In the latter case production can take place at several levels selectively, allowing the field to be developed with fewer wells and therefore more rapidly, but maintenance costs are higher.
It should be noted that that there is a type of completion where tubing is not used. This involves cementing and perforating a small length of casing in place at the level of the production zone. This is appropriate for small gas fields poor in associated liquids and at low pressure.
Once the well has been completed, the wellhead is attached to the top so as to control the flow of fluids (Fig. 2.22). The wellhead is made of:
– The casinghead to which the casing is attached;
– The tubinghead which supports the tubing;
– The Christmas tree which comprises various valves and gauges.
Chapter 2Oil and gas exploration and production
Christmas tree
Casinghead Tubinghead
Figure 2.22 The wellhead.
2.4.3 Well productivity
Well tests are carried out in order to evaluate the productivity or injectivity index of the well, and any damage which may have occurred. These tests together with the results of further laboratory testing will reveal whether any treatment is necessary. The well is then put into service and evaluated. It will subsequently undergo measurements, maintenance, workover or abandonment.
2.4.3.1 The drillstem test
The term drillstem test (DST) refers to all the well testing carried out during drilling. The DST is basically a test intended to establish the production potential of a well and allow it to be characterised.
When a DST is conducted the well is temporarily completed and a special assembly is lowered, equipped with various valves allowing the well to be shut off both at the wellbottom and at the surface, as well as pressure gauges. A sequence of periods of production and obser-vation is defined and the test involves continuously monitoring the pressure of the reservoir during this time. By comparing this with a diagram for different stabilised flow rates, important information is obtained on the depletion of the zone in which the well is producing.
Several production tests are carried out with different wellhead settings in order to obtain production data. This allows certain physical characteristics of the well to be derived, as well as the maximum possible production rate.
2.4.3.2 Methods of stimulation
The productivity of the well, measured in this way, may prove to be poor because of the petrophysical characteristics of the well, or because of damage caused by the drilling. When the natural flow rate of the oil is weak, however, it can be improved by stimulation methods such as acidising or hydraulic fracturing.
Acidising consists of injecting acid which infiltrates the reservoir and dissolves some of the obstructing material. Additives are included to prevent corrosion of the casing or tubing, or blockages resulting from the reaction of the acid with certain types of crude oil.
Hydraulic fracturingis practised in the reservoir in order to open fractures in the reservoir rock by means of high pressure produced hydraulically. These cracks are then wedged open by introducing propping agents such as sand, shells, aluminium balls, glass or plastic.
2.4.3.3 Activation methods
When an oilfield does not contain enough energy to drive the oil up to the surface treatment facilities it is necessary to resort to activation, i.e. either gas injection or pumping. This is necessary in more than three-quarters by number of wells worldwide, although these wells probably account for no more than 20% of world production. When there is an economical supply of gas and the quantities of oil justify the expense, a technique known as gas lift is applied. Gas is injected into the fluid column in a well to lighten it and make it rise as a result of the expansion of the gas. Depending on the production characteristics of the well and the manner in which the gas injection equipment is deployed, the gas can be injected continu-ously or intermittently.
Various different types of pump are used for conventional pumping:
Chapter 2Oil and gas exploration and production
Chapter 2Oil and gas exploration and production Figure 2.23 Pumping jack.
• Sucker rod pump: a downhole volumetric pump assembly driven by a surface recipro-cating action power source via a rod.
• Centrifugal pump: an electrosubmersible pump immersed in the effluent at the bottom of the well, the power being supplied by means of a special cable.
• Hydraulic pump: a downhole reciprocating pump linked to a hydraulic motor.
2.4.4 Well interventions
There are two categories of interventions practised on a well in the production phase: well servicing and workover. These are both intended to maintain or enhance output from production wells.
Well servicing involves the partial replacement of equipment such as downhole pumps, gas lift valves, production tubing and the sealing systems which may fail because of corrosion, waxy hydrocarbons, etc. Well servicing also includes simple operations such as cleaning and sand control.
Workover includes more major repairs such as removal of sand which has intruded into the wellbore and recompleting the well for production from a different zone.