The Merksplas-Beerse well (North Belgium) is a low-enthalpy geothermal production well targeting the Dinantian karstic limestones to a total depth of 1761 m. The presence of methane gas in these limestones generated a particular interest in this well. This paper describes the geological profile of this well and the Dinantian reservoir. The Namurian-Visean boundary at 1630 m is determined by the base of the dipmeter draping pattern in the radioactive Chokier shales (base of the Namurian) on top of the karstified Dinantian limestone. The stratigraphic composition of the transitional interval from Dinantian to Silesian correlates closely to the nearby Turnhout well. The two fractured intervals at 1630-1656 and 1739-1747 m respectively were identified in the Dinantian limestones. They are associated with siliciclastic sections in between pure limestones. The reservoir water is a sodium chloride brine of about 74 °C and at a pressure below the hydrostatic. The water is slightly radioactive because of the contact with the Chokier hot shales. A carbon dioxide gas with methane and nitrogen admixture is dissolved in the water. The gas liquid ratio at standard conditions is about one and the bubble point is around 200-400 psi at reservoir temperature. A long duration pumping test shows a high fracture permeability and a productivity index of 5.4 m3/h/bar with a productivity to injectivity ratio of 1.45.

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Since its introduction in the late 1980's, fracturing of horizontal wells has become a viable completion option. In certain reservoir conditions, horizontal wells offer significant production improvement over vertical wells, however, to maximize their return on investment, it may be necessary to fracture horizontal wells. This is especially true in case of tight gas formations. This paper reviews the technology developed in the area of fracturing horizontal wells. The paper includes discussion on the rock mechanics, the operational, and the reservoir engineering aspects of fracturing horizontal wells. The rock mechanics discussion reviews the various theoretical and experimental work that has been done in the area of fracturing horizontal wells. It also reviews the various phenomena such as creation of transverse and longitudinal fractures, creation of multiple fractures, and fracture reorientation among others that are associated with creation of a fractured horizontal well. Stability of the horizontal well as it relates to stimulation is also discussed. The reservoir engineering portion of the paper discusses the production performance and testing aspects of a fractured horizontal well. Emphasis is given to fracturing tight gas formations, since this area is the one in which this technique is considered to be the most effective. The performance of a longitudinal fracture is examined and compared to a fractured vertical well and to the more popular transverse fractured horizontal well. Because performance of a longitudinal fracture is similar to that of a fractured vertical well, the existing solutions for fractured vertical wells may be applied to longitudinal fractures. This approximation is valid for moderate to high dimensionless conductivity. In the case of transverse fractures, the outer fractures outperform the inner fractures. However, for most cases, more than two fractures are necessary to efficiently produce the reservoir. Operational aspects of fracturing horizontal wells for both transverse or longitudinal fractures are discussed, and advantages and disadvantages of each type will be outlined. Examples and case histories are discussed. The paper also presents guidelines for stimulation of a horizontal well and includes both propped and acidized fracturing as well as matrix acidizing.

This paper was prepared for presentation at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 28 May-1 June 1999.

The tool suite that is introduced in this paper provides Logging-While-Drilling (LWD) formation evaluation with tools rated up to 350°F (175°C) and 25,000 psi (172.4 MPa). These ratings qualify as High Pressure High Temperature (HPHT) environments for LWD services, where the standard ratings are much lower. The HPHT LWD suite consists of measurement-while-drilling (MWD), annular pressure, resistivity and nuclear porosity services. The MWD service provides surveys, optional gamma ray measurements and the mud-pulse telemetry connectivity to the surface. Changes to the real-time pulsed telemetry data streams can be requested from the MWD surface system to accommodate customer requirements for any sections logged within the wellbore. A retrievable MWD system is also available for high-risk drilling environments and conditions. The quartz pressure tool measures annular pressure, drill pipe pressure and mud temperature. The resistivity service comprises of a dual frequency (2MHz and 500 kHz) borehole compensated wave propagation tool with three transmitter-to-receiver coil spacings. The combination of dual frequency, triple transmitter coils and phase/attenuation measurements provides twelve resistivity curves with different diameters of investigation. Advanced resistivity analysis compute six dielectric assumption independent resistivities, and the related dielectric constants. In anisotropic formations in high angle wells, the horizontal and vertical resistivities are calculated. The porosity service is a density-neutron-stand-off-caliper tool. The spectral bulk density (Rhob) measurement is compensated for stand-off. Three neutron porosities are computed from the three neutron detectors. The more environmentally friendly Californium252 source emits a neutron output comparable with a conventional LWD Americium-Beryllium source but at a much lower radioactivity. The source is recyclable, thus removing the need for quasi-eternal storage for depleted sources. The tools in this suite are modular and can be configured in any order. A selection of Bottom Hole Assemblies (BHA) will be shown. Examples will show the range of measurements and the advanced applications. Introduction There is a wide variety of pressure and temperature ratings for the different M/LWD tools on the market1. It appears that ratings above 302°F (150°C) and above 20,000 psi (137.9 MPa) are considered high temperature and high pressure ratings. 350°F and 25,000 psi may not be considered HPHT in some drilling environments, nor would they be considered HPHT in wireline logging tools. In the LWD industry however, where the tools must withstand the temperature and pressure for extended periods of several hundred hours, these ratings indicate HPHT qualification. The M/LWD tool suite that is introduced here is rated up to 350°F and 25,000 psi. Temperature/Pressure Rating The rating is achieved by pre-screening all the electronic components and testing to 350°F. The tools are tested to 25,000 psi. One of the main challenges to meeting the 350°F requirement is the design of electronics. Components have to be carefully selected and individually tested to ensure that they are reliable and capable of operating at these high temperatures. In addition to component testing, rigorous testing of electronics at the system level is necessary to ensure the performance and reliability of the tools at high temperatures. In order to achieve a higher pressure rating, high tensile strength materials are used. Finite element stress analysis of the geometry of the components is also performed and suitable changes are made to accommodate the higher stresses associated with high pressure. The HPHT LWD suite consists of MWD survey, gamma ray and resistivity services for nominal collar sizes ranging from 4 ¾-in - 9 ½-in and in 4 ¾-in nominal collar size for MWD survey, gamma ray, annular pressure, resistivity and nuclear porosity services. The annular pressure service is also available for HP/HT environments in a 6 ¾-in nominal collar size.