Open-hole wells

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. 

Figure 1 Generalized construction of a screened weU and an open-hole well...

The most recent well log summaries for hydrates is that by Anderson et al. (2005) and the log summary in the Mallik 2002 case study by Collett et al. (2005). While a brief overview is presented below and exemplified in the Mallik 2002 case study, the reader interested in details is encouraged to review the above two documents for a thorough exposition of hydrate open hole well logging. 

In the Tbilisi area, the Middle Eocene sequence is a volcanic erupted structure with a very high density of interconnected micro fractures and a relatively smaller number of macro fractures with apertures in the mm range. The matrix rock is of low porosity and low permeability and is not considered to contain any significant reservoir volume. The fields have been producing since 1974 by open-hole wells of which a large number have experienced water break through. 

Batch treatment is also used in treating oil and gas wells. An inhibitor is diluted with an appropriate solvent and injected into the annuals of open-hole wells or into the tubing of gas wells that have a packer. In this application, it is important that the inhibitor contacts all surfaces and that it has good persistence. Most wells require treatment about every 2 weeks. 

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. 

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. 

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. 

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. 

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. 

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... 

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. 

Horizontal completions in unconsolidated formations are being enhanced by a hydrochloric acid (HCl) breaker system for well clean up. Typically, the use of HCl in open-hole environments is avoided because of wellbore stability concerns. However, HCl successfully removes salt fluid loss control materials in wells without noticeable hole collapse [33]. 

In cases where wells have a screened or open-hole interval partially submerged below the water table and a pump is going to be used for both purging and sampling, the pump should be placed at the midpoint between the top of the water table and the bottom of the screen. If different equipment is going to be used for purging and sampling of the well, the pump should be placed at the top of the water column. ... 

The well was originally cased to a depth of 456 m (1495 ft), and the zone was left as an open hole to a depth of 591 m (1939 ft). The depth of the zone has been increased twice. Seasonal injection (fall, winter, and spring) began in late 1966 the system was inactive during late summer. 

The release rate of radionuclides generated inside RC into outboard water depends on the extent of seal failure of its elements, i.e., on the number and size of openings (holes), as well as on specific hydrological conditions within waterborne storage areas (tidal phenomena and ejection caused by rmdercrurents). Radionuclide spreading over water area is principally determined by local peculiarities of tidal phenomena and wind-effected currents. 

Alternatively, the well may be completed open hole, in which the productive formation is not cased or cemented. This type of completion is more likely to be done in consolidated reservoirs where the hole is stable. 

In a cased and perforated well, only a slightly smaller plastic zone exists around the cased hole. In a perforated well in a poorly consolidated sand, the zone just around the wellbore is in a state of plastic stress, and greater flow rates can be attained without failure than will be possible in open hole situations. The production of solids will be governed by the stability of the sand arches behind the perforations. One of the important considerations in reducing solids production is to identify the poorly consolidated layers and avoid perforating them. 

Density. A cement slurry is designed at a density such that the well is under control during the whole process, that is, the static and dynamic pressures remain in between two limits across the entire open hole section during the whole operation. 

A major hydraulic fracturing experiment was performed in September and October 1993 at Soultz-sous-Forets. In the September test, 25,000 m of fresh water were injected between 2850-3350 m at progressively higher rates to 40 1/s and pressures of up to 10 MPa over a 17 day period. In October, a further 20,000 m of water were injected at up to 50 l/s into the entire open hole section. Through the test, it was demonstrated that the fracture network in the basement rock was well developed, with enhanced permeability and a substantial increase in transmissivity (Baria et al., 1999 Evans, 2000). 

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