Open hole

A large investment is made by oil and gas companies in acquiring open hole log data. Logging activities can represent between 5% and 15% of total well cost. It is important therefore to ensure that the cost of acquisition can be justified by the value of information generated and that thereafter the information is effectively managed. 

One method of sampling reservoir fluids and taking formation pressures under reservoir conditions in open hole is by using a wireline formation tester. A number of wireline logging companies provide such a tool under the names such as RFT (repeat formation tester) and FMT (formation multi tester), so called because they can take a series of pressure samples in the same logging run. A newer version of the tool is called a modular dynamic tester or MDT (Schlumberger tool), shown in Figure 3.8. 

The bare foot completion, which leaves an open hole section below the previous casing, is cheap, simple and suitable for consolidated formations which have little tendency to collapse. The slotted liner s an uncemented section of casing with small intermittent slots cut along its length, which prevents the hole from collapsing, but allows no selectivity of the interval which will be produced. The cased and cemented horizontal completion does allow a choice of which intervals will be perforated and produced. None of these examples provides any effective sand exclusion it this is required a gravel pack or a pre-packed liner can be used. 

Hydrocarbon-water contact movement in the reservoir may be determined from the open hole logs of new wells drilled after the beginning of production, or from a thermal decay time (TDT) log run in an existing cased production well. The TDT is able to differentiate between hydrocarbons and saline water by measuring the thermal decay time of neutrons pulsed into the formation from a source in the tool. By running the TDT tool in the same well at intervals of say one or two years (time lapse TDTs), the rate of movement of the hydrocarbon-water contact can be tracked. This is useful in determining the displacement in the reservoir, as well as the encroachment of an aquifer. 

If the drill string becomes differentially stuck, mechanical methods or spotting fluids can be appHed, or the hydrostatic pressure can be reduced (147). In general, penetration of water- or oil-based spotting fluids into the interface between the filter cake and the pipe accompanied by dehydration and cracking results in reduction of differential pressure across the drill string (147,148). Spotting fluids are usually positioned in the open hole to completely cover the problem area. 

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

Figure 8-143. Percent open hole area for perforated and sieve trays.

This relation does not hold for plates having 10% open hole area, as the heights are several times the corresponding heights for 23% and 40% trays at the same vapor rate, Fc. 

Efficiency appears to fall off significantly for open tray areas above 30%. The higher efficiencies are usually obtained in the 20-25% range of open hole area [47]. 

Usually special solids, such as nut shells, limestone, expanded perlite, etc., are added to the drilling muds to fill or clog rock fractures in the open hole of a well. Most of these lost circulation materials can shorten the life of pump parts. They are especially hard on valves and seats when they accumulate on the seats or between the valve body and the valve disc. 

In drill collar annulus, an open hole (use Equation 4-93)... 

Annulus flow around the drill pipe in the open hole section... 

In most air and gas drilling operations, open-hole well completions are common. This type of completion is consistent with low pore pressure and the desire to avoid formation damage. It is often used for gas wells where nitrogen foam fracturing stimulation is necessary to provide production. In oil wells drilled with natural gas as the drilling fluid, the well is often an open hole completed with a screen set on a liner hanger to control sand influx to the well. 

It is quite desirable to place water into an open-hole section prior to running a casing or liner siring and cementing. The water will provide a hydraulic head to hold back any formation gas in the open-hole section that could cause a fire hazard at the rig floor. 

An example of the typical safety problem that can occur when completing these wells is when a 6 in. open hole has been drilled below the last casing (or liner) shoe. The open section of the borehole is usually to be cased with a 4 in. liner. The liner is to be lowered into the open hole on a liner hanger that is made up to 3- in. drill pipe. It is assumed that prior to the liner operation, drilling had been under way thus the stripper rubber in the rotating... 

When controlling a well, be careful not to exceed the fracture pressure anywhere in the open-hole section. An intermediate casing may have to be set to avoid the fracture condition. 

There are also specially designed preventers with shear-blind rams. This type of ram will certainly cut the pipe and seal the open hole. Special precautions should be taken to ensure that the blind rams cannot be mistakenly closed. 

It is required to determine whether or not the formation at the casing shoe will break down while circulating the kick out of the hole. The annulus volume capacity in an open hole is 0.046 bbl/ft. 

Solving the above equation yields p, = 5,561 psi. Since the maximum expected pressure in an open hole is less than the formation fracture pressure, the kick can be safely circulated out of the hole. 

Determine the cementing operation time and thus the minimum thickening time. Assume a cement mixing rate of 25 sacks/min. Also assume an annular displacement rate no greater than 90 ft/min while the spacer is moving through the open-hole section and a flowrate of 300 gal/min thereafter. A safety factor of 1.0 hr is to be used. 

The spacer must have a volume sufficient to give 200 ft of length in the open-hole section. The annular capacity of the open hole is... 

Thus the number of sacks to give that will be 200 ft in length in the open-hole section is... 

The total cementing operation time is somewhat complicated since it is desired to reduce the rate of flow when the spacer passes through the open hole section of the well. The mixing time is... 

Liner. This is a string of casing used to case off the open hole without bringing the end of the string to the surface. Usually the liner overlaps the previous casing string (shoe) by about 200 to 500 ft. 

Most squeeze cementing operations take place in cased sections of a well. However, open-hole packers can be used to carry out squeeze cement operations of thief zones during drilling operations. 

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