Leak-off tests (LOTs) or, preferably, extended leak-off tests (XLOTs), can be successfully used in minimum in-situ stress, S 3 , estimations. Selecting a point on the leak-off graph that represents the best proxy for S 3 can reduce inaccuracies in the use of LOTs as a means of determining S 3 . If the testing procedure is well conducted and recorded, picking the leak-off pressure (LOP) or instantaneous shut-in pressure (ISIP) gives equally valid estimates of S 3 . During testing, most of the pressure applied in the deduction of S 3 is exerted by the static mud column, particularly in overpressured settings where higher drilling mud weights are used. Since the mud column contributes such a large proportion of the applied pressure, estimating S 3 from tests conducted at greater depth means the observed small difference between LOP and ISIP has even less of an effect on the deduced S 3 value. The data used in this study show that LOP closely matches ISIP when considering multiple cycle XLOTs. It can therefore be inferred that the LOP is the fracture re-opening pressure and hence S h given that the assumptions made by the Kirsch equation for wellbore failure are upheld. This study also considers the implications for calculating the magnitude of S H .
SP logs recorded in a West Texas waterflood exhibited enough sensitivity to indicate zones taking fluids in open-hole wells and in wells lined with fiber glass. These surveys are logged both when the wells are shut in and during injection, and the difference in values is the electrokinetic component of the measurable spontaneous potential, which is proportional to the rate of flow into each zone. Introduction In waterfloods such as the Wasson flood in West Texas, uniform injection of water into all productive zones is essential to efficient oil recovery. Wells in the Wasson San Andres field penetrate more than 200 ft of stratified dolomite section. Premature flood breakthrough in a small part of this section can cause producing wells to water out after recovering only producing wells to water out after recovering only a fraction of the total oil reserves. Therefore, profile logs such as flowmeter logs, temperature logs and radioactive-tracer surveys are periodically, run in injection wells so that zones of high intake can be isolated and those of low intake can be selectively pumped into at higher rates, pumped into at higher rates, Conventional profile surveys, however, are generally not reliable in those old wells completed open hole, shot with nitroglycerin, and later cased with an uncemented fiber glass liner. Uncemented liner and hole-size effects confuse the profile interpretations. These problem wells are best surveyed using an unconventional profile tool, the Spontaneous Potential (SP) Log. As implied by Gillingham, zonal differences between SP logs run in wells while they are shut in and during injection are related to flow rate. This technique has the sensitivity to profile most open-hole or fiber-glass-lined fresh water injection wells, but lacks the quantitative accuracy of radioactive-tracer and flowmeter surveys. Field Operation The SP device is the simplest logging tool available. It is basically a recording voltmeter with one electrode lowered inside the wellbore and the other electrode grounded at the surface. In profile logging, the downhole electrode is usually lead, weighted with 10 to 20 ft of insulated sinker bars and machined to pass through 2-in. tubing. The surface equipment includes a standard SP logging panel. The surface electrode consists of a simple electrical connection to the iron wellhead. This round connection is stable and common to most injection wells. Detectable galvanic potentials associated with unlike electrodes (lead and potentials associated with unlike electrodes (lead and iron) are constant on all logging runs in a particular well and are unimportant. The first step in SP profile logging is to survey the well while injecting at a normal stable injection rate. Use of a lubricator allows entering the well without interrupting the flow. The well is surveyed while logging both up and down until the curves repeat within about 3 mv. Runs in one direction occasionally have less noise and drift than in the other direction. In addition, fast logging speeds - greater than 100 ft/ min reduce drift caused by the relative water motion past the down-hole electrode and also reduce the past the down-hole electrode and also reduce the probability of recording, spurious noise. In practice, probability of recording, spurious noise. In practice, the bottom few feet of hole are not logged. Sometimes electrically charged sediment on bottom attaches to the sonde, causing drift as it later washes off. The next step requires surveying the well while it is shut in and stablized. The well is sufficiently stable when the SP signal from a stationary sonde, positioned opposite a zone that was taking fluid, positioned opposite a zone that was taking fluid, stablizes. This zone is chosen from known data or picked by trial and error. picked by trial and error. JPT P. 151
Summary Tough drilling and evaluation problems fall to easy solutions in deep Tuscaloosa wells: Log resistivities in soft shales are linked to impending pore pressure reversals in deep sands; high-resistivity hard shales are linked to lost circulation intervals; and resistivity invasion profiles are linked to freshwater anomalies in boreholes with oil mud. Introduction Several onerous things can happen during the 150 or 180 days it takes to drill a deep Tuscaloosa well in south Louisiana. As an example, consider this scenario: A Tuscaloosa well penetrates the over pressured Chalk formation. The well kicks and subsequently loses returns. The operator sets protective casing and drills ahead into the Eagleford shale. Gas in the mud increases. He sets a liner, drills into the Massive Tuscaloosa, loses returns, and reduces mud weight. Drilling continues. The well penetrates the Interbedded Tuscaloosa and kicks hard. Pipe sticks. The well is sidetracked. Apparent pay zones are tested and flow fresh water. The depth is 18,000 ft (5500 m). The cost: $5 million. But things are not so tough if casing and evaluation programs are based on geomechanical and petrophysical concepts. Understanding this depends on understanding four basic ideas:Formations can be grouped into one of two classifications-soft or hard-based on resistivity logs.Soft (viscoelastic) shales often have high fracture gradients and high pore pressures depending on proximity to massive sands. (Pore pressures can be derived from resistivity logs.)Hard shales (and carbonates) often have lower pore pressures and lower fracture gradients depending on boundary conditions.Fresh water often occurs in isolated, lower-pressure sands, near high-pressure zones containing salt water. Stratigraphy and Shale Resistivity The Tuscaloosa wells penetrate several different stratigraphic units, some hard and some soft (Fig. 1). An outline is given here of the important intervals: Het Lime A hard carbonate and potential lost-circulation zone. Claiborne Group A soft shale interbedded with limes and sands. Not over-pressured except where the Wilcox sands are not developed. Wilcox A sand interbedded with hard, resistive shales. Not developed at the east edge of the trend. Midway A semisoft shale. Shale resistivity is 2 to 3 Omega typical depths, and fracture gradients are high based on breakdown tests. Clayton A thin, soft, over pressured shale. Pore pressures increase with depth as a function of distance from the lower Cretaceous shelf margin (Fig. 2). Chalk A thick carbonate that often is drilled underbalanced. Eagleford A thick, soft, often over pressured shale. JPT P. 428^
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