Oil-Based Mud (VCM)

7.5.1 General

VCM’s are designed to minimise the transfer of water from the mud to the formation (osmosis), by balancing the salinity of the mud to that of the formation, generally by the addition of calcium chloride. The reduction of fluid loss stabilises problem shales and protects sandstones which have reactive clays in the pores. In addition to these benefits there are several others which have reactive clays in the pores. Unfortunately there is a high environmental penalty attached to the use of VCM’s.

• The VCM policy states:

− The use of VCM is confined to the smaller hole sizes, i.e., 12 1_/

4" hole and smaller.

− For offshore operations all VCM cuttings are transported to shore.

− VCM will be used only if a suitable WBM cannot be found to drill the well, from a technical standpoint:

• If the use of WBM may create formation impairment. • In horizontal hole sections.

• In unstable hole sections.

• The proposed usage of VCM must be submitted to SodM prior to the well commencement, and approval granted before it may be used.

• Cost and time savings alone are not sufficient reason to justify the use of VCM. They must be supported by some of the above criteria.

• Reference can be made to the M-I Drilling Fluids Versa Clean Manual for detailed preparation and maintenance instructions on the current VCM system.

• If dry cuttings are being produced over the shakers, an effort should be made to feed the cuttings directly into the cuttings container.

• The cuttings container should be periodically inspected as it is sometimes possible to recover VCM using the mud vacuum.

7.5.2 Safety measures

• Personal protection:

Appropriate gloves, glasses, filters, face masks, eyewash stations, and foot wash stations must be available.

• Rig modifications:

Exposure to VCM vapours and aerosols must be restricted by: − Closed in shakers (sheeting, etc.).

− An extractor fan installed above the shakers. − Mudpits closed off as far as possible.

− All drains/discharge points are to be controlled with fluid content measurements taken prior to discharge.

All enclosed areas must be fitted with air circulation systems capable of keeping the air purity at an acceptable level. The critical factor is the maximum allowable concentration (MAC) valve of mud components (i.e., oil vapour etc.) in the air.

• The following information and warning notes must be displayed on notice boards and on special warning signs at strategic places on the rig while using VCM.

− VCM will be in use during the present drilling programme.

− VCM contains substances that, under certain circumstances, can be dangerous to your health.

− All drilling personnel must take the following precautions:

• Avoid skin contact. This can cause irritation of the skin, (or in the longer term inflammation or eczema).

• Avoid inhaling excessive quantities of vapour or oil mist over a prolonged period. − Modifications have been made on this rig to ensure a safe and healthy operation.

Ventilation is adequate at places where vapours are created.

• All drilling personnel will follow the safety rules associated with VCM: − Wear the prescribed protective clothing and equipment.

− Take the necessary precautions when using materials and equipment to decrease the risk to drilling personnel.

− Contact medical services if you have any health problems.

7.5.3 Company regulations

Reference should be made to the Company H.S.E policy regarding the protection of the environment .

7.5.4 Spacer design - high-density VCM

To achieve efficient removal of weighted VCM’s or invert emulsion fluids, the following spacers have proven to be successful. They have resulted in the minimum amount of waste generated by providing an efficient sweep and the minimum amount of mixing.

1. 5m3 Viscosified water. SG:1.85. 2. 10m3 Seawater + 10 % KLEEN UP. 3. 5m3 Viscosified Water. SG: 1.03.

• To reduce the quantity of contaminated seawater produced displacing VCM, the following technique should be considered. Stop circulation after the chaser behind the VCM is seen at the surface. Allow the remaining oil to segregate and collect in the DP- CSG annulus. This small amount of oil can then be disposed of properly onshore, and the seawater can be disposed offshore if it contains less than the required 40ppm.

7.5.5 Sampling procedures for average oil on cuttings analysis

As all VCM cuttings are shipped to shore it is not an operational requirement that the average oil on cuttings remains below the legal requirement of 100g oil per 1000g dry material (10 percent oil on cuttings).

However, it is important to minimise the volume of cuttings shipped in order to reduce disposal costs and to save mud. The cuttings wash system is now used for this purpose. The wash fluid has been replaced by VCM and the system acts as a mud recovery system, recovering the VCM lost through the solids control system.

Testing for average oil on cuttings is to be performed periodically to ensure that the wash system is operating effectively. To determine the average oil content the following sampling procedure is used. The procedure ensures that a representative sample of the total disposal flow is taken for analysis.

1. When a cuttings treatment system is used with one central/combined exit* point, then only sampling at this exit point is needed.

2. If more exit points exist, then sampling at each exit point must take place. 3. The frequency of sampling while drilling is every hour and the quantity shall be

approximately one-quarter of a litre or half a kilo.

4. After 12 samples have been taken, the samples are homogeneously mixed to

approximately 6kg. From this mixture a sample is taken for the retort analysis. If the bit is pulled before 12 samples are reached, the mixture should be made of the available samples.

5. Utilise lag time (of cuttings to surface) to determine the interval over which the samples have been taken.

6. If samples have been taken from more than one sample point, the mixing of the samples must be in proportion to the quantities produced by the different flows. In order to

determine this ratio it is required to measure the output of each contributory piece of equipment over a period of 5, 10 or 15 minutes, depending on the rate of penetration. 7. Under no circumstances is whole mud allowed to be dumped.

* An exit point is a discharge point to the cuttings transport container. Samples should be taken from the cuttings transport container, at regular intervals, for measuring the oil/mud content of the cuttings. This is to maintain a full check on the mud volume balance.

7.5.6 Hydrogen sulphide

Special considerations must be taken when drilling potentially sour wells. These

considerations cover both personnel and equipment protection. When setting up for a sour well the following areas should be addressed:

• Personnel training: All crews to have H2S training. • Mud selection for the sour zone: Is H2S easily detected? • Ambient air monitors: What is the expected H2S content? • Type of monitoring equipment: H2S and kick detection. • Drillpipe protection: Is it required?

Mud Type Selection

H2S is soluble in many fluids used in the drilling industry. The two most common are water

and oil. The type of fluid being used can affect the ability to detect H2S:

Water:

Water-based muds are preferred for drilling H2S as the H2S is detected more readily as it

comes out of solution at the surface. In addition, a simple HACH test can be used for detecting the presence of soluble sulphides.

Oil:

VCM’s can be used for drilling sour zones, however they must be treated very carefully. H2S is extremely soluble in oil and may not be detected readily. This can make it

extremely dangerous to the rig personnel at the surface due to H2S remaining in the mud

system. One benefit of using VCM is that it coats the drillpipe and substantially reduces the susceptibility of the pipe to H2S attack.

Scavenger Pre-treatment

The decision to add a scavenger as a pre-treatment should be weighed carefully against the following:

1. H2S Content

If only a small amount of H2S is anticipated, and no special detection equipment will be

available, then it may be desirable to scavenge the H2S immediately. In this case only a

small amount of scavenger will be required. When larger amounts of H2S are anticipated

and they are not being scavenged out with simply a pre-treatment, then no pre-treatment should be used. It is much more efficient to treat out larger amounts of H2S with chelated

zinc which will not adversely affect the mud properties. 2. Cost

In most situations very little H2S will actually be drilled due to the hydrostatic overbalance of

the mud. In these situations there is no benefit to pre-treating the mud. The pre-treatment may actually cause more mud problems than the benefit gained through pre-treatment.

Detection Capability

When H2S is expected in quantities that could be hazardous to either personnel or

equipment, it is not possible to pre-treat with enough scavenger to eliminate the problem without causing major problems in mud rheology. In most instances it is usually better not to pre-treat as the mud properties are not affected and the H2S can be readily detected by a

drop in pH at surface, and by the presence of soluble sulphides in the mud. Typically it is better to know that a problem exists than to mask it until it becomes more severe. If pre- treatment is desired then zinc carbonate is typically used due to its low cost. Typical pre- treatment concentrations are 2-5kg/m3.

When H2S is expected in quantities that could be hazardous to either personnel or

equipment, a detection program should be in place. H2S can be detected readily and dealt

with as follows:

1. pH: The pH should be monitored on a regular basis (several times per tour). If a drop in