The technology of the oil industry today is such that more rigid requirements are placed upon cement isolation in wells. In addition, recent advances in well evaluation and completion technology permit completions to be made in zones that would have been considered uneconomical a few years ago. With longer payouts, it is even more important to insure the best possible completion. Many completions are made in formations that depend upon natural fractures, vugs, or induced fractures for commercial production rates. Maximum production under these conditions requires cement isolation before completion attempts to prevent damage resulting from squeezing cement, especially after any well treatment. Acoustic cement bond logging is one of the tools in well completion technology that can be used to insure the best possible completion by insuring isolation of all zones before a completion attempt is made. It shows the degree of isolation. Under many conditions the cost of the log is small in comparison with squeezing, reperforating, refracturing, decreased production, or even loss of a well. Field examples illustrate a number of cases where considerable extra completion expense arose because the information from the bond log was not used. Basic bond log interpretation is included in the Appendix. Introduction Many conditions have placed more rigid requirements upon the effectiveness of cement isolation behind casing. Deeper drilling, with the accompanying higher pressure and temperature, has resulted in higher pressure differentials on the cemented interval. This has required more bonded interval for effective isolation. The increased application of high-volume, high-rate well treatments requires the cemented interval to withstand high pressure differentials. Secondary recovery of all types also requires complete isolation behind casing, both from the standpoint of cost of injected fluids and efficiency of operation. Often, a production test is the most economical evaluation of cement isolation. When there are no other permeable zones near the completion interval, the odds are that isolation is sufficient. In formations of high permeability, when a production test shows channeling from another zone, cement squeezing and reperforating will not greatly reduce productivity. However, there are many other situations in which the economics are such that complete knowledge of the degree of cement isolation should be obtained before perforating. Acoustic Cement Bond Log Application There are many variables that control the effectiveness of the cement in a well depth, temperature, hole size, additives, contamination, type of cement, type of cement flow, etc. Even with the best available cementing program, channels or some other type of unbonded section can exist in critical intervals. Because of these uncertainties, the usage of the acoustic cement bond log has increased greatly in the past few years. The acoustic wave in a cased borehole consists of all arrivals along any coupled path between transmitter and receiver. A recording of this entire acoustic wave, properly interpreted and used (see Appendix), can supply the information needed to design the most economic completion procedure. One possible presentation is the intensity-time recording where dark and light streaks represent the positive and negative half cycles of the acoustic wave. Amplitude is shown by the darkness or lightness of the streaks. The position of the streaks from left to right denotes increasing arrival time. Fig. 1 is an intensity-time recording presentation on a bond log run in 4-in. liner. Only 5 ft of good acoustic bond is indicated above the interval to be perforated. The reservoir pressure at the time of completion had decreased to approximately 5,500 from 9,000 psi originally. Because of high pressure differentials, it was recommended that the liner be squeezed above. However, this was not done, and the well came in producing gas at the rate of 4 to 5 MMcf/D. After several months of production, the well suddenly died. During cleanout operations, sand, shale and cement were recovered from the well. Before the workover was completed, the liner collapsed, and the well had to be plugged. Above the main pay sand there were thin sand stringers at original reservoir pressure that were the probable cause of the liner collapse. In this case, a squeeze before perforating for production and even another bond log run would have been economical. Figs. 2 and 3 show this same acoustic intensity-time presentation on a bond log run in a well that was intended as a triple completion. JPT P. 811ˆ
Laboratory investigations and test-well oscilloscope pictures were made to determine the application of acoustic signals for evaluating the effectiveness of casing cementing. This work indicated the possibilities of measuring the amplitude of acoustic signals from the pipe and from the formation, both in open and cased hole, to indicate the bonding to pipe and to formation. Field tests have shown the applicability of the system, but experience and further model tests have shown that a number of variables affect the interpretation. Some of these variables such as type of cement, quantity of admixes, sheath thickness and time of logging after cementing-are being investigated, and the results to date are included. These results indicate that it will be possible to evaluate the effectiveness of cementing under most conditions. Introduction When production tests do not agree with log, core-analysis or other formation evaluation data, the effectiveness of casing cementing is usually questioned. Even in wells which have been produced for some time and then begin to produce water or excess gas, the isolation by cement behind the casing has been found to be incomplete by a squeeze and reperforation. The answers to these questions in the past have been quite expensive and often damaging to formation productivity during the process of repair. Considerable research effort recently has been expended by service and producing companies on the cement bond log, which offers the possibility of evaluating the effectiveness of cementing. The process involves measuring the amplitude of an acoustic signal, with the quality of the cement bond (plus some other variables) affecting the signal amplitude. At this point a definition of the term "bond" should be established. For the purpose of this paper, a "bond" is defined as a coupling which joins either the pipe and cement or the cement and formation in such a manner that no intervening space is present and no density discontinuity exists except from steel to cement or from cement to formation. Acoustic Cement Bond Logging Experimental Investigation To completely evaluate the possibilities of determining the effectiveness of cement isolation with acoustic signals, a series of test holes was set up with various cement conditions using ideal Portland 15-lb/gal cement. Using an acoustic logging tool, oscilloscope pictures were taken of various spaced-receiver signals under a number of conditions. As expected, it was found that the signal through the pipe depended upon the cement bond to the pipe. Also, the amplitude of the formation signal depended upon the cement bond to the formation. The pipe signal can be considered as a vibration of the pipe which has an arrival time of 57 microseconds/ft, the bonded pipe having a very low-amplitude signal because of the damping effect of the cement sheath. The amplitude of the formation signal depends upon the presence of a bonded path through which the acoustic energy can travel. If the cement is not bonded to the formation, the signal must cross four additional solid-liquid interfaces, with the resulting loss of energy at each. Of course, some energy will be lost because of the density variation existing at the cement bond-to-formation interface, but this loss will be much less for the unbonded condition. Some of the scope pictures from this study (3 ft between the transmitter and the receiver) are presented in Figs. 1 and 2. Fig. 1(A) shows the high-amplitude pipe signal from uncemented pipe, with a very low amplitude formation signal super-imposed on the ringout of the pipe vibration. Views B and C of Fig. 1 show the same low amplitude in bonded pipe, although the section of pipe in View C was sandblasted to remove mill scale, etc. In both B and C, the high-amplitude formation signal indicates the good bond to the formation. Views D, E and F of Fig. 1 show different widths of channels and the thicknesses of cement sheaths. This and other data indicate that the thickness of the cement sheath affects the amplitude up to about a 2-in. sheath (for Portland cement); but after that point, essentially no change in amplitude can be distinguished. The channels in these three pictures were rectangular, which probably is not too common in the field; the more common situation is decentralized pipe, with insufficient cement between the pipe and formation on one side (a condition which will be shown later). JPT P. 1093^
An investigation was initiated into the mechanics of gas-well perforating because of a number of cases of drastic increases in productivity of gas wells after reperforating the same interval. Since these reperforations were with gas at low pressure in the wellbore, a test series was set up in a flow laboratory to investigate the factors involved. Berea sandstone was perforated with gas, kerosene as a formation fluid. Tests were made over a wide range of pressure differentials both to the formation and in the wellbore. The results of the test series show that the fluid pressure conditions in the wellbore at the time of perforating are much more important for gas-filled than for liquid-filled formations. For liquid-filled formations no plugging occurred in the cores tested that could not be removed by fluid flow from the core. A practical completion technique has been developed and put into practice to control the environmental conditions while perforating gas wells. Introduction Many gas wells do not come up to expectations of deliverability based upon open-hole drill-stem tests or by comparison with other wells completed in the same reservoir. This discrepancy between expectations and actual deliverability is more common with gas wells than with oil wells. High damage ratios, indicating severe skin damage, are also quite commonly observed in gas wells which have been completed with the "normal" completion methods, where wellbore pressure is maintained above reservoir pressure at the time of perforating. In many states, gas-well daily allowable is based on absolute open-flow potential. With the rapidly increasing demand for gas at more attractive prices and a ready market. it becomes much more important to obtain the maximum potential and deliverability from each well completed in a reservoir. Evidence of Poor Completions in Gas Wells Gas wells, which in comparison to nearby wells had not potentialed at a satisfactory rate, have often been reperforated. High-pressure control equipment was placed upon the well-head and entry made against pressure with a through-tubing gun. The new perforations were usually in the same interval as the original perforations, and the results were vastly improved potential tests for the well. The consistent success in improving deliverability of these gas wells indicates that the "normal" completion technique produced results which were inferior to the technique used in reperforation. A factor in the reduced deliverability of gas wells is apparent in the problems associated with blast joints. Numerous observers in the field and in the technical literature have repeatedly pointed out that where a blast joint is employed, seldom more than one to four shiny spots or holes are ever seen on the blast joint, regardless of the number of perforations in the casing. This indicates, of course, that from one to four holes produce the entire amount of gas coming from this upper completion. Presumably the normal completion technique left the remainder of the perforations ineffective. Several laboratory investigators have simulated the normal oilwell completion technique of wellbore pressure higher than formation pressure and an extended time of filtration into the formation after perforation. The tests showed that the flow efficiency of the perforations was reduced under these conditions. These results led to considerable improvement in the flow efficiency of oilwell completions. However, all of the reported tests have been made with a liquid in the test sample. In view of this. and the problems on gas-well completions, a series of tests on gas-saturated samples was started to determine if a significant difference exists in the results of perforating gas-bearing formations as compared to oil-bearing formations. Laboratory Procedures and ResultsThe objectives of these tests were to determine the relative effects of pressure conditions and fluid conditions in the wellbore at the time of perforation of gas-saturated samples in comparison to the previous work done on oil-saturated samples. JPT P. 647ˆ
Well logs which present the full wave train of the acoustic signal show variations in both the compressive and shear wave arrivals with lithological changes. This information, along with elastic constants, which can be derived from the logs, can be used in the design techniques available for well stimulation planning. This information has been successfully applied in one area of California to select completion intervals that could be successfully stimulated at pressures below wellhead limitations. Additionally, pressures below wellhead limitations. Additionally, this data could be applied to the prediction of fracture height and of the fracture closure or sand embedment problem where productivity increases from well stimulation are only temporary. Introduction Optimization of well treatment over recent years has necessitated more exact input data regarding the formation characteristics. Design techniques have been developed which require specific rock properties; however, many of the properties used properties; however, many of the properties used are assumptions or average values obtained from limited core samples. Variations of geological factors under downhole conditions are readily observed on many types of logs; however, many of these variations are seldom taken into account in treatment design. Their influence on vertical fracture height is an important consideration for which there has been nothing very consistent in the method of prediction.
Laboratory data and field results have shown that casing perforations are plugged when there is a pressure differential toward the formation at the time of perforating. As a result, there has been a trend toward thru-tubing completions so that the wells can be perforated with a pressure differential toward the wellbore. However, there has been an accompanying sacrifice in depth of penetration because of the smaller charge size, especially in the thru-tubing hollow carrier guns. Also, when the encapsulated charges are used, the casing has to absorb the shock when the gun is fired. Recently, a perforating system has been developed whereby the deep penetration of the larger charges in casing guns can be combined with the efficient clean-out of the negative differential perforating. In this system each perforation is sealed perforating. In this system each perforation is sealed off from hydrostatic pressure as the gun is fired. The volume inside the gun at atmospheric pressure is sufficient to contain the products of combustion and still give essentially full formation pressure as a differential into the gun. The resulting implosion into the gun insures that each perforation will have maximum capacity for flow or injection. Laboratory and field data are presented showing the benefits of this new perforator. Full allowable wells are being obtained through natural production in many cases. In others lower breakdown pressures, higher injection rates, and more efficient well treatments and sand consolidation jobs are the result. Introduction The effectiveness of perforations in oil and gas well casing has been the subject of a number of papers since the introduction of bullet guns in 1932 and of jet guns in 1948. In 1956 Allen and Worzell and Krueger reported the results of perforating Berea sandstone targets with various perforating Berea sandstone targets with various wellbore fluids and pressure differentials. Their data indicated that serious perforating plugging occurred whenever there was a differential pressure to the formation at the time of perforating, especially with mud in the well bore. These plugs consisted of crushed formation, liner particles, jet charge particles and mud. Pressure differentials as high particles and mud. Pressure differentials as high as 430 psi were required to initiate backflow on some of these tests.
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