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Well depth
The depth of a well is measured by two different methods as drilling proceeds. The driller measures the length of all the components of the drill string (called the drillers tally) and adds them up to calculate the measured depth whilst the wireline logging engineer records the length of cable plus logging tools used to reach the bottom of the well. Due to stretching and bending of the equipment the drillers tally and the wireline measurements rarely exactly agree. Broadly speaking, for a 3,000m well the true measured depth may be plus or minus 3m from the depth that is recorded by each of these methods.

All depths are recorded with respect to a depth reference (datum). On offshore rigs mean sea level (MSL) is normally the standard for geological evaluation. However, a working measured depth reference (MD) is used whilst drilling, normally taken from the top of the rotary table. This is designated MDBRT (measured depth below rotary table) or MDBKB (measured depth below kelly bushing).

All measurements made from this level have the difference between the height of the RT/KB and the MSL subtracted to get the true vertical depth (TVD subsea or TVOss) in straight wells. Formation tops, pressures and other measurements are quoted at both MOBRT and TVOss depths on the composite log of the well. For deviated wells the TVOss needs to be calculated from the MDBRT. Surveying instruments, run at intervals whilst drilling, and/or monitoring-while drilling tools, run behind the bit, measure azimuth (from horizontal) and inclination (from vertical) to map progress. With these numbers the TVOss of the well and its co-ordinates at each depth level can be calculated.
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Horizon depth

The expected depths at which certain geological horizons will be drilled are commonly called prognosed formation tops, normally quoted in TVOss and TVOBKB and estimated from nearby wells and seismic data (using velocity analysis of the rock layers through which the seismic waves have passed). They will include the depth at the seabed and at all the key horizons in the section to be drilled. In well-known areas all of the horizons

will be named (for example, the Base Cretaceous Unconformity in the northem North Sea) as will the top and bottom of the reservoir section (for example, the Rotliegend Sandstone in the southern North Sea).

In less well known areas some or all of the formation tops will be referred to names given to key reflectors mapped on seismic data (usually colours, for example, the Blue Horizon). The reservoir section might be expected over an interval between two reflectors, perhaps determined from reservoirs recognised in distant offset wells or from outcrop data onshore. Clearly accurate geological predictions of depth and rock type in exploration wells will help reduce the cost of the well as the engineering programme can be more closely specified. All other things being equal the more wells in an area, the more accurate the prognoses.

Total depth

The planned total depth of a well is critical to its cost since deeper wells mean longer drilling times and more equipment. Moreover, as depths increase so formations get harder and drilling becomes slower. Of course, some formations drill much faster (for example, salt) at whatever depth so this is a general rule only, but in all cases trip times and potential drilling problems will increase.

In addition the depth at which each formation in the geological section appears and the expected depth of the target reservoir or reservoirs are critical to the drilling programme.

In particular, the cost and effectiveness of the proposed casing programme rests upon forecasts by geologists, using seismic data and offset wells where available, of the depth of hazardous layers such as shallow gas zones, over-pressured shales and fractured layers that may be prone to lost circulation. The total depth of a well (TO) is thus predefined so that the correct amount of equipment can be purchased or hired in advance.

Except for a few exploration wells drilled in relatively unexplored sedimentary basins, which may be drilled to economic basement (the depth below which no reservoirs can ever occur), the TO of a well is usually just below the base of the deepest expected hydrocarbon-bearing reservoir. Globally, average TOs range from 2,500 to 3,500m but, of course, this varies enormously both within individual sedimentary basins and within countries and regions. Exploration wells drilled in the deep waters of the Gulf of Mexico for example, may reach 6,000m to 8,000m. Global drilling depths are increasing, especially as gas, which can exist at much greater pressures and temperatures (hence depths) than oil, gains in importance
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