Borehole

The pores between the rock components, e.g. the sand grains in a sandstone reservoir, will initially be filled with the pore water. The migrating hydrocarbons will displace the water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need to be in communication to allow migration, and also need to allow flow towards the borehole once a well is drilled into the structure. The pore space is referred to as porosity in oil field terms. Permeability measures the ability of a rock to allow fluid flow through its pore system. A reservoir rock which has some porosity but too low a permeability to allow fluid flow is termed tight . 

For the very first section of the borehole a base from which to commence drilling is required. In a land location this will be a cemented cellar in which a conductor or stove pipe will be piled prior to the rig moving in. The cellar will accommodate the Christmas tree (an arrangement of seals and valves), once the well has been completed and the rig has moved off location (Fig. 3.13)... 

If a shallow kick off in soft formation is required (e.g. to steer the borehole away underneath platforms) a technique using jet bit deflection or badgering is employed (Fig. 3.16). A rock bit is fitted with two small and one large jet. With the bit on bottom and oriented in the desired direction the string is kept stationary and mud is pumped through the nozzles. This causes asymmetric erosion of the borehole beneath the larger jet. Once sufficient hole has been jetted, the drill bit will be rotated again and the new course followed. This process will be repeated until the planned deviation is reached. 

The chemistry of cement slurries is complex. Additives will be used to ensure the slurry remains pumpable long enough at the prevailing downhole pressures and temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the drilling of the next hole section. The cement also has to attain sufficient compressive strength to withstand the forces exerted by the formation over time. A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake and thereby achieve a better cement bond between formation and cement. 

Some clay minerals may absorb some of the water contained in the drilling mud. This will cause the clays to swe//and eventually reduce the borehole size to the point where the drill pipe becomes stuck. Prevention mud additives which prevent clay swelling e.g. potassium salt. 

Residual stresses in the formation, resulting from regional tectonic forces may cause the borehole to collapse or deform resulting in stuck pipe. Prevention sometimes high mud weights may help delay deformation of the bore hole. 

During drilling operations sometimes large volumes of drilling mud are lost into a formation. In this case normal mud circulation is no longer possible and the fluid level inside the borehole will drop, creating a potentially dangerous situation as described below. The formations in which lost circulation can be a problem are ... 

A low strength formation in which open fractures are initiated by too high mud pressure in the borehole... 

In the event of a sudden loss of mud In an Interval containing overpressures the mud column in the annulus will drop, thereby reducing the hydrostatic head acting on the formation to the point where formation pressure exceeds mud pressure. Formation fluids (oil, gas or water) can now enter the borehole and travel upwards. In the process the gas will expand considerably but will maintain its initial pressure. The last line of defence leff is the blowout preventer. However, although the BOP will prevent fluid or gas escape to the surface, closing in the well may lead to two potentially disastrous situations ... 

If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

The basic data gathering methods are direct methods which allow visual inspection or at least direct measurement of properties, and indirect methods whereby we infer reservoir parameters from a number of measurements taken in a borehole. The main techniques available within these categories are summarised in the following table ... 

In the pre-development stage, core samples can be used to test the compatibility of injection fluids with the formation, to predict borehole stability under various drilling conditions and to establish the probability of formation failure and sand production. 

The sidewall sampling tool (Sl/VS) can be used to obtain small plugs (2 cm diameter, 5 cm length, often less) directly from the borehole wall. The tool is run on wireline after the hole has been drilled. Some 20 to 30 individual bullets are fired from each gun (Fig. 5.35) at different depths. The hollow bullet will penetrate the formation and a rock sample will be trapped inside the steel cylinder. By pulling the tool upwards, wires connected to the gun pull the bullet and sample from the borehole wall. 

In a more recent development a new wireline tool has been developed that actually drills a plug out of the borehole wall. With sidewall coring (Fig. 5.36) some the main disadvantages of the SWS tool are mitigated, in particular the crushing of the sample. Up to 20 samples can be individually cut and are stored in a container inside the tool. 

In preparation for a field wide quick look correlation, all well logs need to be corrected for borehole inclination. This is done routinely with software which uses the measured depth below the derrick floor ( alonghole depth below derrick floor AHBDFor measured depth , MD) and the acquired directional surveys to calculate the true vertical depth subsea (TVSS). This is the vertical distance of a point below a common reference level, for instance chart datum (CD) or mean sea level (MSL). Figure 5.41 shows the relationship between the different depth measurements. 

The most oommon method for measuring formation resistivity and henoe determining hydrocarbon saturation is by logging with a resistivity tool such as the Laterolog. The tool is designed to force electrical current through the formation adjacent to the borehole... 

Permeable intervals can be identified from a number of logging tool measurements, the most basic of which is the caliper tool. The caliper tool is used to measure the borehole diameter which, in a gauge hole, is a function of the bit size and the mudcake thickness. Mudcake will only build up across permeable sections of the borehole where mud filtrate has invaded the formation and mud solids (which are too big to enter the formation pore system) plate out on the borehole wall. Therefore the presence of mudcake implies permeability. 

If oil and water are mixed as an emulsion, dehydration becomes much more difficult. Emulsions can form as oil-in-water or water-in-oil if mixed production streams are subjected to severe turbulence, as might occur in front of perforations in the borehole. Emulsions can be encouraged to break (or destabilise) using chemicals, heat or just gentle agitation. Chemical destabilisation is the most common method and laboratory tests would normally be conducted to determine the most suitable combination of chemicals. 

Providing the land surfaoe above a reservoir is relatively flat, it is generally cheaper to drill and maintain a vertical well than to access a reservoir from a location that requires a deviated borehole. In unpopulated areas such as desert or jungle looations It is common to find that the pattern of wellheads at surface closely reflects the pattern In which wells penetrate the reservoir. However, in many eases constraints will be planed on drill site availability as a result of housing, environmental concerns or topography. In such conditions wells may be drilled in clusters from one or a number of sites as elose as possible to the surface location of the reservoir. 

With this technology even boreholes, up to 2mm underneath the surface, can be identified, A remarkable borehole is represented in illustration 10, For the elucidation of the temperature contrast, a three-dimensional temperature distribution of the entire blade is shown beside the infrared picture (the similarity of the temperature distribution with the actual blade airfoil is purely coincidental). 

The large temperature difference of the remarkable borehole, opposite other boreholes and their environment is significant. This high temperature difference is a typical feature for a small wall thickness between borehole and blade surface. For technical reasons, precise eroding of the boreholes is difficult. Due to this, the remaining wallthickness between the boreholes and the blade surface has to be determined, in order to prevent an early failure, Siemens/Kwu developed a new method to determine the wallthickness with Impulse-Video-Thermography [5],... 

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