Evaluation work for an exploration well will be carried out from the Central Offices, so it is essential that the staff are kept up-to-date about what is happening in the field, and receive the data that they require, in the format they require.

1 General

The Drilling manager should have a face-to-face meeting with Central Offices staff prior to spudding to discuss the requirements, with special attention to identifying the essential data and that which is "nice to have". Once operations have commenced direct contact should be maintained between the Head of Drilling/Drilling Engineer and the staff member responsible for the evaluation.

Similarly the staff member responsible with designing DSTs/ Production Tests should be kept informed of the progress of the well.

2 Mud-logging

The mud-logging service comprises:

• the collection, description and packaging of cuttings and cores,
• the measurement and reporting of geological and drilling data,
• the production and presentation of interpreted data.

Details of the standard requirements should be included in the scope of work of the mud-logging contract. Specific requirements such as sampling intervals will be given in the Well Proposal. Prior to finalising the invitation to tender for mud-logging services this should be discussed with Central Offices who can advise on the latest views on methods, quality control and the application of incentives. The invitation to tender should specify what experience must have the mud-logging geologists and if they are already aware of the Company procedures.

A mud-logging unit for an offshore unit must be pressurised. For a land location this may not be a requirement; however, since the location layout will not be known at the time of tendering for services, a pressurised unit should always be specified to allow for its positioning within a hazardous area.

If "slim-hole" drilling is planned it will be necessary to specify the provision and use of a differential flow kick detection system in the contract. These systems are expensive and are justifiable for conventional sized holes if limited inflow volumes during a kick, as a result of rapid detection, would allow a casing string to be omitted.

Mud-logging data presentation will be on VDUs in the unit, but it is recommended to have a terminal in the office of the Company supervisor and at the drillers console.

A requirement of the contract is the presentation of the data in digital format on completion of the well. Since a high quality telecommunications link will almost certainly be set up for the transmission of wireline logging data, the mud-logging service company should be asked to provide a link to their computers, and mud-log data transmitted at the same time as wireline logging data to the recipients of the latter. Alternatively, an electronic "fax-formatted" mud-log which can be sent by fax over the normal link.

3 Wireline logging

The logging programme will be specified in the Well Proposal. This is unlikely to be available at the time of tendering for the contract, which should therefore make provision for the availability of a full suite of standard logging tools.

Although the Well Proposal will not be available some basic information will have to be agreed and given to the service company before they can prepare a tender. This includes an indication of the possible location(s), the approximate well depth, the minimum hole size, and whether horizontal drilling is foreseen. If there is a possibility of high temperatures and/or pressures this should also be mentioned when inviting tenders, as should the proposed mud-type.

Some of the tools must be permanently on site under an exclusive service agreement and some must be available within a certain time on a call-off basis. The choice of which tools to keep on site will be a function of the time required for transport from the service company's support base (including an allowance for weather conditions if relevant), the rental costs and the probable daily operating cost.

Regardless of the evaluation tools, a small selection of DP perforators should be included in the equipment, as well as DP and DC cutters, and free point indicator equipment.

High quality data link will be set up for wireline log data to be transmitted. Provision of the appropriate interface hardware should therefore be included in the wireline logging contract.

Wireline operations will probably include side-wall sampling and several runs with a wireline test tool, plus data transmission, all of which will take a considerable amount of time. Contractual provision should be made for limiting the number of hours for which any one engineer or other crew member (in particular the winch man) is continuously on duty. Any additional engineer could very usefully be a specialist in wireline sampling work.

A "well-shoot" will almost certainly be required during the final logging job on each well or before for correlation purpose. Offshore no special provisions are required. Onshore, a special pit may have to be dug to suspend the airgun. If possible, this should be included in the scope of work of the civil engineering contract.

The pit must be a minimum of 50 m from the well (typically the distance is 75 m) and a maximum of 100 m from the logging unit. A pit 5 m x 5m x 5 m is required for a Vertical Seismic Profile , but 3 m x 3 m x 3 m is sufficient for a check shot. The pit should be filled with mud. If possible the pit should be on the opposite side of the waste-pit from the well - if that is not possible a trench should be dug between the well-shoot pit and the well.

If the ground is too soft to dig a pit of the required size, it should be lined. A convenient method is to use large diameter culvert sections. Alternatively a steel box section can be constructed and buried; to improve the acoustic coupling the box should have no base. If it is preferred to line the pit with cement rather than bury a steel box, the size should always be greater than 5 m x 5 m or the shock waves from the gun will destroy the walls and the gun will be buried. Driving casing into the ground does not give good results.

4 Measurement/formation evaluation while drilling (MWD/FEWD)

There are two types of measurement for which MWD is applicable:

• Deviation surveys to be made at frequent intervals without running tools on wireline.
• The measurement of drilling parameters such as weight on bit, torque, mud-motor rpm and also temperature.

The basic petrophysical evaluation data can all be measured while drilling; gamma radiation and resistivity, radioactive porosity and density and sonic velocity.

MWD equipment is complex and relatively fragile (the order of magnitude of the mean time between failures is just over a week), however most failures can be rectified on site by the operator.

It is recommended that MWD is only used if a positive requirement can be identified. The principal situations which will justify MWD/FEWD in exploration are:

• For deviation surveys in situations when the directional control required (either by legislation, company policy or geological reasons) leads to surveys at such short intervals that the cost of conventional surveys plus the required time would be greater than the cost of MWD.
• For formation evaluation logs in situations where early information is required. However it should be remembered that the MWD sensors are still several metres above the bit, and if mud motors are being used they are ten to twenty metres above it. When looking for a critical coring point at a formation top the most efficient method will still be to stop and circulate bottoms up at drilling breaks.
• For formation evaluation logs while drilling pilot holes for shallow gas detection. FEWD logging of a pilot hole will enable the top and bottom of a gas-containing interval to be identified so that either the hole can be plugged back and casing set above the gas or to give confidence that the gas interval has been completely penetrated and will be cemented off behind casing.

If the company is operating in an area where there is information from reference wells that the borehole is unstable, the use of FEWD will allow data to be obtained even if wireline logs cannot be run.

If MWD is used it should be a type which allows normal progress to continue while the mud pulses are being transmitted. However for all MWD tools the string has to be held stationary while making deviation surveys. Other measurements are made while drilling continues normally.

A disadvantage of MWD equipment is that it has a restricted temperature rating (125°C to 150°C maximum).

5 Coring

The circumstances under which cores should be taken must be specified so that the well-site staff are clear about what is required of them. It is advisable to avoid the situation of having to discuss the situation between rig and office every time a possible coring point is reached.

6 Testing

General

The decision whether or not to test an exploration well will be made as a result of the exploration geologists' request for information about the reservoir. It should be clearly established from the start what the objective(s) of the test is/are, which in most cases is only to establish the type of reservoir fluid plus pressures, with no time being spent on the acquisition of non-essential, but "nice to have", information. If that is the case a Drill-Stem Test without production to surface is all that is required. To obtain a complete reservoir description production to surface will be necessary.

The three fundamental components of a Drill Stem Test are a retrievable packer, a downhole valve assembly (the DST tools) and drill-pipe to surface.

Production Test equipment consists of a permanent type packer with tubing to surface, with or without DST tools, plus a set of surface equipment.

It is current practice NOT to have surface test equipment on site. However contractual arrangements should be in place so that mobilisation of the equipment, which weighs of the order of 160 tonnes (excluding tubing), to the area can be initiated without delay.

Given that DST equipment has much lower rental rates than Production Test equipment it may well be decided to have it available on site during the drilling of the objective horizons. Otherwise a set of tools should be made the subject of a call-off contract. The criterion will be the estimated transport time from the contractor's base.

If it is already known in the preparation phase that there is a high probability of having to carry out an extended production test, for example if the well is part of a farm-in arrangement in an area where hydrocarbons have already been identified, then the situation is different and arrangements can be made to have the Testing equipment on site at the appropriate time (which may well be by having it on site from start-up, depending on transport availability).

Drill-stem testing

The standard down-hole valves are operated by means of annulus pressure, which implies that a cased hole is required above the packer. They are thus suitable for cased-hole or barefoot completions. They can be used both on- and off-shore.

Open-hole tests (ie with the packer set in open hole) are also possible using pipe manipulation to operate the valves. This cannot be done from a floating drilling unit.

Although the principal use of DST equipment is for inflow tests, it can still be considered for a test involving flow to surface. Since the surface equipment will usually take a considerable time to obtain on site (with the possibility of shipping tubing and down-hole equipment at the same time), the advantage of Drill-Stem Testing over Production Testing is reduced compared with the case of an inflow test. However there remain several advantages of a DST over a "Production Test":

• The saving in the rig time required to lay down DP and pick up tubing and vice versa, before and after the test respectively.
• The possible elimination of the necessity to mill out a permanent packer after the test.
• When using a land-rig or a jack-up the possible elimination of the necessity to run a string of casing for testing purposes only.
• When using a land-rig or a jack-up the possibility of open-hole testing successive reservoirs as they are drilled. This may allow drilling to be terminated earlier than would have been the case otherwise. (Especially if this has been foreseen and the surface equipment is already on site, so that a good reservoir evaluation can be made).
• Not insignificant is the saving due to the elimination of tubing rentals.

A DST is not advised for long tests, more than say two or three days, and not for high surface pressures, more than say 3000 psi.

Production tests

A Production Test is much more flexible than a DST. It is a definite requirement for a long test and for high tubing pressures.

A Production Test can be used with a "Barefoot Completion" so long as the assembly does not extend below the casing shoe, or there is confidence that the formation is competent.