PROCESS FOR PREDICTING THE PERFORMANCE QUALITY OF AN OIL BORE HOLE CEMENTATION AND EVALUATING THIS PERFORMANCE

Abstract: The invention concerns the cementaion of an oil bore hole or of similar bore holes. According to the invention, the theoretical acoustic impedance of cement is evaluated as a function of some theoretical cementaion parameters, this impedance is corrected as a function of the actual meeasured density of the cement slag during pumping, a correlation is established between with the depth as a function of the data recorded during cementation and of the bore hole geometry, and a value Z = f (depth) is deducted therefrom, which gives access to the prediction of the accoustic atteenuation curve : CBL (prediction ) = f (depth) corresponding to the parameters of the actual cementation. By comparison with the (measured) CBL curve, the exact zone where a secondary or repair cementation will be necessary is deducted.

Description: The present invention concerns the field of oil, gas, or water, geothermic, injection or production bore hole cementation, or the like. It is known that, after boring a hole, for instance for oil or gas, a metallic tubing is lowered before cementing the ring-shaped zone thus created between the rock and the tubing. The cement has the functions of preventing a collapse of a bore hole under the rock pressure and to insulate the various underground formation zones. It is known that the cement slag is injected through the inside of the tubing (behind an inert fluid) towards the bottom of the bore hole, after which the cement raises in the ring-shaped zone and flushes the drilling mud. Obviously, the quantity of cement slag is computed in such a way that the inside of the tubing is not cemented (in this tubing, the inert fluid is placed). Those techniques, as well as the many related products and compositions, are perfectly known to those skilled in the art. The problems encountered at the time of enacting these techniques also are well known. In particular, it may happen, at some places in the bore hole, that cementation is not perfect as a result of an imperfect movement of mud, of an unforecast fluid loss towards a very permeable or fractured underground formation, of a tubing asymetry etc.. Now, it is extremely important that cementation is as perfect as possible on the whole useful height of the bore hole to avoid any "weak point" or "communication" in the cemented ring-shaped zone. It thus is completely usual, after a cementation operation, to check the cement quality over the whole useful height of the bore hole by means of a well known bottom tool which is lowered into the tubing. This tool analyses the propagation and attenuation of (very generally acoustic) waves and produces an answer curve called "logging" (or more commonly "log") the analysis of which by an expert allows detection of the zones where, for some reason, there exists a weak point in the cement : mud pocket, poor cement/rocks or cement/tubing bond, etc. This curve is commonly called "CBL" ("Cement Bound Log" or cementation quality curve). The expert then recommends a second cementation process of those zones ("repair"). All of those operations are absolutely known in all their details and it is useless to explain them here in a more detailed fashion (see SPE 16 652, 1987) It is also known that secondary effects as well as parameters that cannot up to now be usefully treated can affect the precision of the log. This is absolutely unavoidable in view of the nature of operations which are conceived and controlled from the surface, but which take place at the bottom of a hole in conditions which, obviously, cannot be 100% guaranteed. It thus can happen that some log anomalies cannot be correctly interpreted, which, for instance, might lead to recommending for reasons of safety cementation curing processes, some of which can in fact be useless. Since each of those secondary or curing cementation processes is extremely costly for the bore hole operator (delay in production, cost of operation itself, etc.), it is easy to conceive the enormous interest which would be presented by a process allowing to predict with precision the log that should be obtained after cementation taking into account all available parameters of the operation and of the bore hole. Namely, such a "predictive" log would allow, by comparison with the "actual" log obtained after cementation, to locate the zones where there really exists a weak point in the cementation, without the possibility of the log interpretation being mislead by the unavoidable "secondary effects" mentioned above, (or only in a minimal proportion ). This interest of course was perceived for the last 20 or 30 years, but the factors able to influence the log are so many, so complex, and sometimes so uncertain, that no predicting log exists up to this date. The invention allows reaching that object. It was demonstrated that the attenuation of acoustic waves emitted (then collected) by the tool allowing the cementation log (CBL) tracing is a direct function of the cement acoustic impedance (see SPE 16 652). It also was shown that the thickness of the cement at a given point is one of the most critical factors. It shall be recalled that the acoustic impedance Z of the cement is the product of the density ρ of the cement by the propagation speed of the acoustic impulse in the cement (UPV), i.e. the compression waves speed in the cement. Reference can also be made to the documents SPE 16 139 and 18 028. A crucial step has consisted in managing correlating Z, with an excellent approximation, with some of the very many parameters of the cement, the bore hole, the underground formations, and with the elapsed time (tc) between the time where the cementation has been performed and the time where the cementation log is traced. The major problem was to obtain a good approximation in spite of the number and complexity of the parameters, and the imprecision which compusorily affects some them, and to manage that this approximation can be used to reach the object of the invention. An approximation was chosen which is rendered by the following equation : Z c = K x ρ c x X + a bX + c (I) Z = theoretical acoustic impedance of the cement = ρ.UPV K = f (tc, cement set up time (TT), temperature) ρ c = cement density (kg/ℓ) computed for the enacting of the cementation operation X = P (cement slag porosity) x Y (slag volume obtained from a cement bag, i.e. 94 pounds or about 43 kg) a, c : are constants established by the approximation. The appended figure 1 illustrates the curves obtained for two different cement types at two different temperatures. It is quite surprising to ascertain that it was possible to reach an approximation which is 1) independant from the cement type (which obviously is an absolute imperative for a possible usage on the various oil fields in the world, where different cement types compulsorily are used as a function of the supply sources, of climatic conditions, etc.) and 2) that can be used (see below) as a starting point for the predictive log, which is the object of the invention. Finally, the second crucial step of the invention, in combination with the previous step Z = f (ρ,X), consisted in discovering that there exists a relationship again independant from the cement type and, further, independant from the temperature between ΔZ and Δρ where Δρ = difference between ρ c forecast at the time of the cementation operation conception ("job design") and ρ m measured in real time during cementation; and ΔZ = difference between Z c computed from ρ c and Z m computed from ρ m (by equation (I) above). Although the oil cements present large and well known differences according to their type, their brand, etc, it can be seen on the appended figure 2 that the relationship Δz = f (Δρ) can be represented by two rectilinear segments, independently from the temperature and the cement type. Since Δρ can be accessed by real time recording of the actual density during cementation by means of a data acquisition apparatus ("PACR"), the relationship represented on figure 2 allows accessing to Δz and consequently correcting the z value approximately computed from the formula (I) on the basis of the theoretical cementation parameters. The actual value of Z can thus be accessed. On the other hand, the correlation between Δρ and the bore hole depth is known, by taking into account the bore hole geometry (diameter, etc.) which is known and the pumping flow as a function of time, which also is known by surface measure. Z actual = f (depth), can thus be accessed, i.e. (since it is known that the acoustic attenuation is a function of the impedance) the "predictive" cementation "log" CBL (the log should be obtained, after taking into account all important parameters thanks to the above steps) can be accessed to, if the cementation quality is perfect. When this predictive log is compared with the actual log obtained by measure in the bore hole by a known bottom tool, it is possible to locate without error the zones where a second repair cementation, potentially, really is necessary. It thus is possible to avoid secondary cementation at locations in the bore hole where, in the previous logs, the unforecast and unavoidable secondary elements could induce a mis-interpretation. The whole of the operations are schematized in the appended figure 3. Finally, the appended figure 4 represents a cementation operation report. The density, curves reponse to γ radiation and CBL, come from an actual cementation operation performed in 1986, except for the "predictive CBL" curve which recently was traced in laboratory by the above indicated process on the basis of the actual file data from 1986, and superimposed on the 1986 curves. The γ reponse curve was kept for simplicity, but it is not used here. As for the predictive "CBL" curve, only the useful portions (zones A, B, C, D and E) appear to avoid overloading the "actual CBL" curve. "Conception" indicates the theoretical curve which should be followed during the cementation operation; "PACR" indicates the real time recorded curve by a data acquisition apparatus, during operation; CBL actual = CBL curve actually obtained by the bottom tool after the cementation operation; CBL forecast = theoretical CBL curve obtained from the conception data (prior art CBL); CBL predictive = theoretical "corrected" CBL curve, obtained by the method according to the invention. It can be seen that the usual comparison of the "actual" CBL and of the "forecast" CBL manages to delimiting 5 zones A to E where the deviation between the two curves can be interpreted as a zone where the cementation requires a repair. On the other hand, the comparison of the "predictive" CBL curve obtained according to the invention with the same "actual" CBL allows showing that, in fact, only the zone E can present a repair problem. This namely is the only zone where after the correction method according to the invention, a substantial deviation still exists between both CBL actual and CBL predictive curves.

Claims: Process for predicting the performance quality of a bore hole annulus cementation operation in an oil, gas, water, geothermic, injection or production or the like bore hole, characterized in that : a. the theoretical acoustic impedance Z c of the cement is computed as a function of the cementation conception (or "job design") parameters and date; b. the difference Δρ between the theoretical cement slag density forcast at the time of the conception and the actual, real time measured, density, during cementation, is evaluated by a data acquisition apparatus; c. a correlation between Δρ and Δz is established, where Δz represents the difference between Z c computed under a) and the actual cement value Z m ; d. Z m is computed by Z m = Z c + Δz where Δz is computed by means of the correlation established under c); e. a correlation is established between Δρ and the depth at a given point of the bore hole taking into account the known geometry of the bore hole and the real time recorded cementation parameters from the data acquisition apparatus; f. by means of the correlation between Δρ and the depth, the correlation between Z m ("predictive" actual value) and the depth is established; this correlation in fact is a logging or "log" ("CBL") representing the theoretical quality of the cementation corrected as a function of the actual cementation parameters; g. the "predictive" CBL obtained under f) is compared with an actual CBL log recorded by an appropriate bottom tool; h. the zones, where the deviation between the "actual" CBL and the "predictive" CBL is sufficient to be interpreted as indicating a zone where a repair cementation might be necessary, are determined. Process according to claim 1, characterized in that in step a) Z c is obtained by : Z c = K x ρ c x X + a bX + c (I) where : K = f (t c , TT, T) where : t c = elapsed time since cementation TT = cement setting time T = temperature ρ c = cement density computed for the cementation operation process X = P x Y where : P = porosity of the cement slag Y = cement slag volume obtained from a "bag" (about 43 kg, 94 pounds) of cement. a and c are constants established by approximation. A process according to one of the claims 1 or 2, characterized in that the relationship Δz = f (Δρ) may be represented by two rectilinear segments.

Inventors: JUTTEN, JACQUES
Applicants: PUMPTECH N.V.
Application year: 1989
Main classification: E21B47
IPC codes: G01V1, E21B47