Q1. a) At depths of high signals of GR (high content of clayed minerals) use oil based mud because water based mud will make the clay formations to swell.

At depths of low signals of GR (very low or no clay minerals) use water based mud because there are not implications using this type of mud due to nature of the formation.

b) Exploration stage

First well drilled is the exploration well, a wildcat. To determine If hydrocarbons are present, if so, reserves and recovery are estimated followed by a proposal

Appraisal stage

Further wells drilled to determine the commercial viability of the field and whether it should be developed

c) (i) The spontaneous potential (SP)

Records the naturally occurring electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid and shales. Low signal means sandstone minerals and high signal means shale or clay minerals.

(ii) Neutron Log

The Neutron Log is primarily used to evaluate formation porosity, but the fact that it is really just a hydrogen detector should always be kept in mind. It is used to detect gas in certain situations, exploiting the lower hydrogen density, or hydrogen index. High signal indicates high porosity and low signal means low porosity.

(iii) Density log

Is a porosity log that measures electron density of a formation. Dense formations absorb many gamma rays, while low-density formations absorb fewer. Thus, high-count rates at the detectors indicate low-density formations, whereas low count rates at the detectors indicate high-density formations.

e) Effective porosity is the interconnected pore volume or void space in the rock that contributes to the fluid flow or permeability in the reservoir. Total porosity is the total void space in the rock whether or not it contributes to fluid flow.

Q2. a) 1. Mud returns from the subsurface through the well containing the drilling solids, equipment included: gumbo removal.

2. Large and small particles of drilling solids contained in the mud are removed by shale shakers, desender, desilter, etc.

3. Addition of surface drilling fluid into the circulating mud, equalizers and stirrers are included in this section.

4. It is the last part of the surface system. Most of the usable surface volume should be available in this section. Processed and treated fluid is available for various evaluation and analysis procedures just prior to the fluid recirculating downhole. It should be mixed, blended and well stirred. Equalizers and stirrers are included in this section as well.

5. Treated drilling fluid is injected back into the drill pipe, equipment included: pumps.

b) ROP is the speed at which the drill bit can break and perforate the rock down the surface. Factors: Hard formation, depth and temperature on the bit.

c) (i) The general functions of all casing strings are:

1. To furnish a permanent borehole of precisely known diameter through which subsequently drilling, completion and producing operations may be conducted.

2. To allow segregation of formations behind the pipe which prevents interformational flow and permits production from a specific zone.

3. To afford a means of attaching the necessary surface valves and connections to control and handle the produced fluids.

(ii) The purpose of perforating the casing is to allow contact between formation fluids and the well so that fluids can flow through from the formation to the well bottomhole and up to the surface.

d) Sandstone consists of fragmental materials such as quartz (very good porosity and permeability). Limestone consists of carbonate minerals (low permeability and high porosity). Shales consist of very fine minerals such as clays, mudstone, mica etc (very low K and porosity).

Q3. b) Stuck pipe is a situation whereby the drill string cannot be rotated and/or moved up or down. Possible cause: some clay minerals may absorb some of the water in the mud. This will cause the clays to swell and eventually reduce the borehole size to the point where the drill pipe becomes stuck. Prevention: use mud addictives which prevent clay

from swelling. Another possible cause: excessive pressure differentials between the borehole and the formation. Remedy: reduce mud weight, continuously rotate or move the string.

Lost of circulation is when large volumes of drilling mud are lost into a formation. Remedies: moderate loss; addition of lost circulation material to the system such as mica flakes or coconuts chippings, sudden loss in a hydrostatically pressured interval; drill ahead without drilling mud but with large quantities of surface water to cool the bit, sudden loss in overpressured interval; remediated by reducing the mud weight.

Q4. b) Directional Drilling

Directional drilling is the ability to steer the drill-stem and bit along a planned trajectory to a desired bottomhole location.

Directional drilling allows multiple production and injection wells to be drilled from a single surface location.

Allow drilling into reservoirs where vertical access is difficult or not possible. For instance an oilfield under a town, under a lake, or underneath a difficult to drill formation

Horizontal Drilling

Horizontal drilling is commonly defined as the drilling of any well in which the lower part of the well bore parallels the oil zone.

Allow production from thin oil-rim reservoirs, avoid drawdown-related problems such as water/gas coning, early gas/water breakthrough

Can be used in many situations where conventional drilling is either impossible or cost prohibitive.

Extended Reach Drilling (ERD)

Extended-Reach Drilling (ERD) is essentially an advanced form of directional drilling, it employs both directional and horizontal drilling techniques

Allow to eliminate additional platforms and use land-based drilling in fields that normally would require costly offshore operations.

Allow to reach a larger area from one surface drilling location, or to keep a well from a longer distance in a reservoir in order to maximize its productivity and drainage capability

Formation pressure is the pressure exerted by the formation fluids within the pore spaces of the rocks. Its relationship with rig safety is that if there is an abnormal or over normal formation pressure there is the risk of fluid blow out at the surface.

Techniques to detect abnormal pressure while drilling:

Drilling rates, gas in mud, etc D – Exponent Dc – Exponent MWD – LWD Density of shale from cuttings

The primary and secondary indication of kicks is under normal formation pressure.

The reason to drill a non vertical or deviated well is to achieve or reach the target zone in thin formations or when the well is placed at a distance away from the targeted formation and also to avoid drilling through gas caps or when there is an obstruction such as salt dome in vertical drilling operations.

Underbalanced drilling is performed when the drilling mud weight in use is lower than the equivalent mud weight required for the drilling operation. The problem is that if it is too low it can cause blow out.

Leak of test

Run and cement casing

Drill out ~ 10 ft below the casing seat

Close the BOPs

Pump slowly and monitor the pressure

The purpose of the leak of test is to find out the fracture pressure, fracture gradient pressure and therefore determine the maximum allowable mud weight to drill the well hole sections.