SPE/IADC Members Abstract We review the conflicting literature on gas migration velocities during kicks. We consider the laboratory and large scale test data that shows that for any local gas void fraction of more than 10%, the influx migrates at approximately 100 ft/min. We also review the evidence from field experience that shows that gas can migrate much more slowly (the typical rule of thumb suggests that gas bubbles move at approximately 15 ft/min) and in some cases remain stationary. We show that the yield stress of the drilling mud which holds cuttings in suspension whilst making connections, can also hold gas bubbles in suspension, and report an experimental study of these gas suspension effects. Significant volumes of gas can be held in suspension during a gas kick, this trapped gas remaining stationary until the mud is circulated out of the well. We consider the implications of this for well control operations, and present field data where gas was injected into a marine riser, it dispersed and remained stationary until circulated out. We show that a single bubble migration model, which neglects gas suspension, predicts that as the gas rises and expands it unloads the riser. By simulating the gas suspension characteristics we model the field data. We conclude that gas in moderate concentrations (more than 10%) migrates quickly, typically at 100 ft/min. This migrating influx leaves a trail of suspended gas in the mud that remains stationary. For small kicks in deep wells the entire influx can be distributed, at a low concentration, and remain in suspension until the gas-cut mud is circulated out of the well. Gas Migration Velocity - Literature Controversy There is a major controversy in the published literature over gas migration rates during kicks while drilling. Experimental tests in small flow loops and in test wells show the gas migration velocity is around 100 ft/min, while field estimates suggest that gas rises at around 15 ft/min or more slowly. We consider this discrepancy. Johnson and White showed that, in typical drilling geometries, in realistic drilling fluids, for gas concentrations larger than 10%, gas migration velocities were around 100 ft/min, significantly larger than the equivalent migration rates in water. The viscosity of drilling mud hinders the bubble break-up process allowing gas to migrate as bigger bubbles (which travel faster). They also observed that the yield stress of the drilling mud would hold low concentrations of gas in suspension with no migration. Rader et al. reported similar results for gas migration in a 3.7 m, (12 ft) flow loop and a 1800 m (6,000 ft) well. The gas velocity in the well was measured using the time of flight principle. Hovland and Rommetveit reported large scale tests in a 1500 m (5000 ft) deep test well, which had a maximum deviation of 630. They used the time of flight between pressure transducers mounted at different depths in the well to measure a gas slip velocity of 0.55 m/s (110 ft/min). A widely accepted "rule of thumb's used in the field says that gas bubbles migrate at 0.085 m/s, (15 ft/min). Blount claimed that he had evidence of gas migration rates of around 0.014 m/s (3 ft/min), although he did not specify how these were derived. In field situations an accurate estimation of gas migration during a well control incident is very difficult. P. 93
Summary. Field drill-off test results are compared with data from laboratory simulations. A simple theory for analyzing drill-off tests is developed. The weight-on-bit (WOB) decay with time is close to exponential, but large threshold WOB'S, resulting from poor weight transmission downhole, are sometimes observed in field tests. Introduction The drill-off test was devised nearly 30 years ago as a practical means of estimating and optimizing drilling performance, but its use in the field has been limited. Two factors seem to limit its value: the quality of data recorded with conventional rig equipment and uncertainty in data interpretation. A joint program was undertaken by Sedco-Forex and Schlumberger Cambridge Research to address these limitations and hence to increase the information actually obtained from a drill-off test. The work involved two aspects: the development and implementation of a rig-site drill-off test data acquisition system and the laboratory simulation of drill-off tests. This paper describes the progress to date, concentrating on the comparison of field and laboratory drill-off test data and on the resultant improvements in our interpretation of drill-off tests. Procedure The drill-off test is a simple, practical procedure, proposed by Lubinski, for determining the relationship between rate of penetration (ROP) and surface WOB. The driller builds WOB to a predetermined maximum and then sets the brake. As the bit drills, the WOB decays at a rate determined by the ROP and the drillstring compliance. The surface WOB is recorded as a function of time. The drillstring compliance is determined from a knowledge of the drillstring composition and is used with the recorded WOB to compute the distance drilled as a function of time. Differentiation yields the ROP as a function of time, and crossplotting gives the dependence of ROP on WOB. Ref. 2 describes an alternative procedure for the analysis of drill-off test data based on Vidder's procedure. The times taken to drill off successive increments of 4,000 lbf [ 18 kN] WOB are recorded, and the mean ROP over each increment is computed from the drillpipe compliance. (Note that length changes in tool joints and drill collars can be neglected.) A log-log plot of ROP against WOB reveals the exponent relating ROP to WOB. If there is a significant threshold WOB-i.e., a WOB below which the bit will not drill-then the exponent can be determined by subtracting different threshold WOB's from the recorded values until a straight-line fit is obtained. Regardless of which analysis is adopted, variations in rock drillability and oscillations in the hook load can combine to restrict the meaning of the derived relationship between ROP and WOB unless some method of smoothing the raw data is used. The next section presents one way of overcoming the problem. When a drill-off test is designed, both the distance drilled and the rate of change of hook load must be considered if the test is to generate results that are representative of the formation being drilled. Assuming that no change occurs in hydraulics during the drill-off, the change in length of the drillstring is related to the change in WOB by ..........................................(1) With an 8 1/2 -in. [21.6-cm] roller-cone bit, the maximum change in WOB is restricted to at)out 45,000 lbf [200 kN], and the bit would normally be run with 5-in. [ 12.7-cm] drillpipe. The compliance of the bottomhole assembly (BHA) is negligible compared with that of the drillpipe. Therefore, the following equation gives the length of formation drilled: ..........................................(2) The rate of change of WOB with depth is ..........................................(3) Initially regarding 5,700 lbf/in. [1 kn/mml as a reasonable upper limit for the rate of change of WOB and assuming 600 ft [200 m] of drill collars, we calculated a minimum drillstring length of 3,000 ft [ 1,000 m]. Thus, the maximum length of formation drilled during the drill-off will be 8 in. [20 cm]. As a result of these calculations, we decided that it was unlikely that valuable information would be derived from drill-off tests performed at depths of less than 3,000 ft [1000 m], and we restricted our investigations accordingly. Interpretation Theory Laboratory and field studies have shown that it is reasonable, in many circumstances, to consider the ROP, R, proportional to the WOB, W. Thus, ..........................................(4) During a drill-off test the WOB is linked to the bit ROP by the elasticity of the drillstring. ..........................................(5) ..........................................(6) Integration gives the following expression for WOB as a function of time during a drill-off test. ..........................................(7) We found in some cases that the WOB decays not to zero, but to a finite offset or threshold WOB, W,. If this threshold WOB is introduced into the expression relating ROP to WOB, the dependence of WOB on time becomes ..........................................(8) Three valuable pieces of information can be obtained by fitting this expression to WOB data recorded during a drill-off test: the quality of fit, which indicates whether the assumption of linearity between ROP and WOB is valid the value of W, whose significance is discussed below; and the value of K. which characterizes the ROP. Note that the curve-fitting scheme adopted to determine W, is equivalent to the trial-and-error procedure previously mentioned because both rely on finding the value of W, that gives the best fit between the data and the assumed relationship between ROP and WOB. Here, however, we work with data recorded at a much higher frequency and consequently gain enhanced resolution. Experimental Details More than 50 drill-off tests were performed in vertical wells and in wells deviated up to 40deg.
No abstract
The search for oil has required, and without a doubt supplies, a tremendousamount of information on the structure, composition, physical properties, andhistory of sedimentary rocks. The earliest and? most complete source ofknowledge is the sample or core of the rock itself, the uses of which are wellknown. The purpose of this paper is primarily to discuss a new method forobtaining these cores, and to illustrate its use in connection with electricallogs. Most cores have been obtained through a special adaptation of rotary drilling, and the results have been remarkably good, considering the mechanical problemsinvolved. Unfortunately, because of the time and expense, this method cannot beused for the entire length of the drill hole. Even though ingeniouscontrivances have been evolved in order to reduce the time spent in roundtrips, such as wire-line coring equipment, mechanical coring still remains tooexpensive to be carried out continuously. In practice, therefore, one isfinally compelled to estimate the probable depths of the more interestinghorizons and, allowing a wide margin of safety, to core through those sectionsonly. It is true also that interesting results have been obtained by examiningthe cuttings brought to the surface by the mud circulated in the well duringthe process of drilling. Be that as it may, the inescapable fact remains thatkey horizons may be passed inadvertently, for one reason or another, while thedrilling operations are being performed. The only continuous record of theformations encountered is that furnished by electrical surveys. Obviously, it would be desirable to make the electrical survey first and totake the samples from the wall of the hole according to the indications on theelectrical log. In this way, it would be possible to core only the exactsections desired, eliminating the otherwise inevitable estimates as to thepoint at which to start coring, and thereby reducing to a minimum thisexpensive operation. At the same time this method would provide a maximum ofuseful samples. A device for taking samples of the formation from the walls of a hole thereforeoffers the possibility of recovering cores from zones otherwise lost as asource of this type of information. T.P. 1062
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