SPE Members Abstract Experiments were conducted to study how sonic and ultrasonic cementation logs are affected by microannuli. By creating both gas- and liquid-filled microannuli of known sizes at the casing-to-cement interface, measurements could be recorded and analyzed. Depending on the nature of the fluid and the size of the microannulus, results showed that all cementation logs were sensitive to microannuli. Gas at the casing-to-cement interface had a very strong effect on the ultrasonic measurement but had minimal effect on traditional sonic Cement Bond Log (CBL) measurement if the microannulus was in the range of one m. CBL measurement was very sensitive to water-filled microannuli. Ultrasonic measurements were capable of evaluating the quality of the cement behind a water-filled microannuli as large as 100 m. Liquid-filled microannuli could be identified through the large coupling attenuation measured with multi-spacing sonic tools. Analysis of these experiments brings new insights to cementation-log interpretation. Introduction Interpretation of cementation logs is without any doubt subject to controversy. For over 30 years, CBL measurements have been used to evaluate cement jobs. More recently, special tools have been developed to overcome some of the limitations of the traditional sonic measurements. Use of ultrasonics, has made it possible to improve cement evaluation, mainly through a spatial resolution which allows a much clearer identification of incomplete mud removal. However, using higher frequency waves renders the signal more sensitive to the local events, such as poor pipe-surface condition. In many cases, field experience and theories have helped people to better understand the response of the tools to specific situations. When a small gap exists between the casing and the cement, the response of acoustic and ultrasonic tools is affected. There is a microannulus effect which renders cement job evaluation even more difficult. Several articles have been published to explain how a microannulus can be created or induced and how this affects cementation logs. Temperature or pressure changes are the most common causes of microannuli (see Appendix 1 for usual formulae). P. 505^

The objective of this paper is to describe and quantify the influence of cement slurry composition on the mechanical properties of the hardened cement and on the main output of the cement bond log: attenuation of the first arrival amplitude. Laboratory experiments were performed in ambient conditions of pressure and temperature, with a laboratory type cement bond tool coupled to an oscilloscope and a data acquisition unit. The tool was immersed in oil filled, 4.5 inch, 11.3 lbm/ft casing. The annulus between the casing and a 9.5 inch PVC pipe backed by air was filled with the cement slurry. The following mechanical properties were determined as a function of time: compressive strength, shear bond strength, total chemical contraction, velocity of compressional waves and resonance frequency. More than 20 different cement slurry formulations were tested, with densities ranging from 10 to 19 lbm/gal, and containing as widely different additives as bentonite, soluble silicate, silica microspheres, hematite, salts, latex, dispersant and fluid loss agent. Amongst many results and correlations, several conclusions can be drawn. The relationship between compressive strength and velocity of compressional waves, verified for most slurries, is not valid for salt containing slurries, when for all the systems tested, a single relationship exists between Young's modulus and Poisson's ratio. Particulate extenders like silica microspheres provide, for a given slurry density, higher acoustic impedance and CBL attenuation rate than chemical extenders like soluble silicates. Finally, latex in the cement does not influence the CBL. Introduction Cement job evaluation is mainly based on the interpretation of acoustic logs, like the Cement Bond Log (CBL). In the early 60's, some theoretical work [1] stated the CBL attenuation rate was related, amongst many parameters, to the cement density and velocity of compressional and shear waves through the cement. Experimental work was performed at the same time, leading to the construction of a nomograph well-known as " CBL interpretation chart". This single chart could not be used to evaluate every cement job and has been later modified to take foamed cements into account [2]. Later developments in logging [3] give access to the direct measurement of the acoustic impedance of the material located immediately behind the casing, with an angular distribution which enables to locate mud channel when cement and mud acoustic impedances are different. It seems now obvious that the knowledge of acoustic properties of the cement will improve the evaluation of cement jobs through a better interpretation of acoustic logs. However, the use of ultrasonic methods to characterize oil well cements is fairly new [4]: most of the recommended methods used in the oil field, like compressive strength determination, are destructive, thus little acoustic data exist which could help for cement job evaluation. Ultrasonic methods, which have been widely used for more than 40 years in the concrete industry [5], offer one major advantage over traditional methods: they are non destructive and can be used "in situ". Furthermore, ultrasonic properties of a material are directly related to its elastic properties.

Summary This paper describes and quantifies the influence of cement slurry composition on the mechanical properties of the hardened cement and on the cement bond log (CBL). Laboratory experiments were performed on more than 20 different cement slurry formulations with densities ranging from 1200 to 2280 kg/m3 [10 to 19 lbm/gal]. Results show thatCBL attenuation rate is directly related to the acoustic impedance of the cement;particulate extenders like silica microspheres provide, for a given slurry density, higher acoustic impedance and CBL attenuation rate than chemical extenders like soluble silicates;latex in the cement does not influence the CBL;the relationship between compressive strength and velocity of compressional waves, verified for most slurries, is not valid for salt-containing slurries; anda single relationship exists between Young's modulus and Poisson's ratio. Introduction Cement job evaluation is based mainly on the interpretation of acoustic logs, like the CBL. In the early 1960's, theoretical work showed that the CBL attenuation rate was related to, among many parameters, the cement density and velocity of compressional and shear waves through the cement. Experimental work was performed at the same time, leading to the construction of a nomograph known as the CBL interpretation chart. This single chart could not be used to evaluate every cement job and has since been modified to take foamed cements into account. Later developments in logging gave access to the direct measurement of the acoustic impedance of the material located immediately behind the casing, with an angular distribution that enables mud channels to be located when cement and mud acoustic impedances are different. It now seems obvious that the knowledge of acoustic properties of the cement will improve the evaluation of cement jobs through better interpretation of acoustic logs. However, the use of ultrasonic methods to characterize oilwell cements is fairly new; most of the recommended methods used in the oil field, like compressive-strength determination, are destructive, so few acoustic data exist to aid cement job evaluation. Ultrasonic methods, which have been widely used for more than 40 years in the concrete industry, offer one major advantage over traditional methods: they are nondestructive and can be used in situ. Furthermore, the ultrasonic properties of a material are directly related to the elastic properties. This is not the case in cube-crushing tests, which do not directly measure any fundamental property. In particular, the propagation of a compressional wave through a material and its natural resonance frequencies are dependent on the elastic modulus of the material. During cement hardening, both mechanical strength and elastic modulus increase. This can also be seen by measurements of pulse velocity or natural resonance frequency. Spinner and Tefft proposed methods to determine mechanical resonance frequencies of cylinders and bars and derived equations to compute elastic moduli from natural resonance frequencies, density, and geometric characteristics. A standard test method was subsequently defined that covers the measurement of fundamental transverse, longitudinal, and torsional resonance frequencies for calculating dynamic moduli and Poisson's ratio. Equations show that even when Poisson's ratio is unknown, a good estimate of Young's modulus can still be made from measurements of natural resonance frequency. This is not possible with the pulse-velocity measurement because the relationship between Young's modulus and sound velocity is influenced much more by the Poisson's ratio. Studies have been carried out to relate pulse-velocity measurements to the strength of the concrete. Elvery et al. found that the relationship between ultrasonic pulse velocity and cube strength is practically independent of the water/cement ratio and temperature between 1 and 60 degrees C [34 and 140 degrees F] but is influenced by the aggregate content and the type of cement. The method has also been used successfully to monitor the influence of various additives on the hydration of the cement. Sturrup et al. concluded that pulse-velocity/strength relationships could be established for concrete at early ages, but that pulse velocity is rather insensitive to even major increases in strength at later ages and should not be used for matured concretes. Furthermore, pulse velocity can also be affected by cracks, voids, or other discontinuities in the concrete. In the course of the work described here, extensive testing with both destructive and nondestructive techniques has been conducted on a large variety of cement formulations. The purpose of this paper is to show how cement slurry composition affects various physical and acoustic properties of set cement and the CBL. It shows also how the data generated and cross correlations made between different types of measurements can improve cement job evaluation. Experimental Setup and Procedures CBL Measurements. The experimental setup, which was used previously to study the influence of borehole geometric parameters on the CBL output, can be divided into three sections.The mixing area is where the volume of slurry required for the test is prepared. It is located 3m [10 ft] above the test cell so that the slurry can flow by gravity. The slurry is kept under agitation with a high-shear paddle until its rheology becomes constant and is then pumped by gravity into the test cell.The test cell is a 1.2-m [4-ft] section of sandblasted casing (114-mm [4.5-in.] OD, 102-mm [4-in.] ID) positioned vertically in a 241-mm [9.5-in.] -diameter PVC pipe backed with air but providing enough cement thickness to have a CBL peak measurement free of reflections. The annular space is cemented, while the inside of the casing is full of low-viscosity oil where the sonic sonde is located.The sonic sonde consists of a cylindrical body in Teflon holding four ceramic transducers arranged along the axis with a 0.152-m [0.5-ft] spacing. These transducers, used on standard field sonic sondes, have a ringing frequency of about 17 kHz [17,000 cycles/sec]. The sonic sonde, which is kept centered inside the casing, is fired 5 times/sec, and the sonic signal received is displayed on a digital storage oscilloscope. The amplitude of the first peak, A1, of the sonic wave is monitored vs. time, and the attenuation rate of the signal can be derived from the following relationship: where = attenuation rate, dB/m [dB/ft], Ls = transmitter/receiver spacing, m [ft], SPEPE P. 77^

Summary We intend (1) to show that formation characteristics affect the cement bond log (CBL) signal in most wells, (2) to prove that high-amplitude CBL's obtained in concentric casing-string configurations are often an artifact resulting paradoxically from the excellent paradoxically from the excellent quality of the cement job, and (3) to propose procedures for running CBL's propose procedures for running CBL's in concentric casings to eliminate this artifact. Experiments performed to study the influence of the cement/ formation or cement/external-casing interface show that, in all cases, these interfaces induce perturbations on the received waveform. The strength of these perturbations depends mainly on the impedance contrast at the interface. Field logs confirm the laboratory results and clearly show that high CBL amplitudes in well-cemented concentric casings are an artifact that could easily be eliminated by appropriate setting of the measurement windows. Introduction Primary cementing is one of the most critical and most difficult operations Primary cementing is one of the most critical and most difficult operations in the life of an oil well. Considerable effort has been and is still being devoted to improving all aspects of cementing, including postjob evaluation. Early postjob evaluation methods consisted exclusively of either pressure testing the casing or locating the top of cement behind the pressure testing the casing or locating the top of cement behind the casing. Since the development of CBL's in the early 1960'S, it has been possible to quantify the results of a cement job, within certain possible to quantify the results of a cement job, within certain measurement limitations, throughout the cemented interval. Recent experimental work proved that CBL attenuation rate is related more directly to the cement acoustic impedance than to the cement compressive strength. Classic CBL interpretation requires that the cement-sheath thickness be sufficient to ensure that no energy reflected from the cement external interface is superimposed on the measured Peak El amplitude. Until recently, 19 mm [0.75 in.] was considered a sufficient cement-sheath thickness; however, Ref. 6 showed that the necessary minimum thickness is a function of the acoustic properties of the cement. True values of minimum cement-sheath thickness can vary from 25.4 to 76 mm [1 to 3 in.]. Because these values are much larger than the average annular gap observed on most production strings, it is clear that in most cases soundenergy reflections at the cement external interface affect the CBL signal. This paper presents the results of extensive experimental work performed with realistic annuli. The influence of the cement external performed with realistic annuli. The influence of the cement external interface is investigated as a function of tool spacing, cement, and formation characteristics. Formations were simulated with cement formulations of known acoustic properties. In the case of concentric casings, experimental results are confirmed by field logs. From an analysis of these results, a simple modification of the CBL running procedure is proposed. This procedure effectively eliminates problems procedure is proposed. This procedure effectively eliminates problems related to narrow cement-sheath thickness in concentric casings. Experimental Setup Test Slurry. Cemoil API Class G cement was mixed at 1. 9 kg/L [15.8 lbm/gal]. The mixing procedure was as described in Ref. 6; a high-shear paddle was used to mix the slurry for 25 minutes before pouring the paddle was used to mix the slurry for 25 minutes before pouring the slurry into the annular gap. A sample was kept for pulse velocity measurements. The thickening time of this cement slurry was around 6 hours, estimated from the temperature increase inside the casing. During setting, pulse velocity measurements of the slurry were found unreliable, as shown in Ref. 7. For this reason, cement acoustic impedance is given only for times exceeding 8 hours. Synthetic Formations. These were simulated with special cement slurry designs. Three different slurries were designed and prepared to have lower or higher acoustic impedance than the test cement slurry. This was achieved by using an appropriate water/cement ratio combined with the proper amount of weighting agent or extender. After mixing, the slurry was proper amount of weighting agent or extender. After mixing, the slurry was poured into the annular space between a 160-mm [6.3-in.]-OD PVC pipe and a poured into the annular space between a 160-mm [6.3-in.]-OD PVC pipe and a 310-mm [12.2-in.] -ID steel pipe. A sample of each slurry was kept for acoustic impedance measurements. When the cement was set, the PVC pipe was removed, leaving a hollow cylinder of synthetic formation 75 mm [2.95 in.] thick. The complete assembly was then kept saturated with water for at least 3 months. This ensured that the synthetic formation material was fully hydrated with stable properties for the final tests. Table 1 gives the relevant properties of each synthetic formation at the time of the tests. The weakest formation (Test F3) had an acoustic impedance of 3.45 Mrayl (see SI Metric Conversion Factors), similar to some unconsolidated shales. The remaining two formations (Tests F1 and F2) had acoustic impedances of 8.3 and 9.5 Mrayl, respectively, comparable with a reservoir rock with 25% porosity. Test Cells. These consisted of casing sections 1.67 m [5.5 ft] long immersed in a water bath 1.52 m [5 ft] deep. The water bath can hold up to seven test cells at a temperature controllable between 20 and 70 degrees C [68 and 158 degrees F]. JPT P. 1158

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOil and service companies have made massive efforts to minimize the risks associated with driving in Nigeria. Despite these efforts, most companies still record major accidents, including fatalities. Recently, a major service company implemented an active journey management system that has proved very effective in eliminating driving-related fatalities and accidents. The journey management system combines specific training, the use of driving monitors and risk-reward schemes for all the drivers.Major companies operating in Nigeria recognized the exceptional driving performance record resulting from this system in 2000. Subsequently, several companies initiated a cooperative program to implement the same journey management system. All participants observed a rapid driving performance improvement. The trend of the number and severity of all categories of driving-related accidents from all participating companies is shown. It compares favorably with statistics from other geographical areas and demonstrates that commitment and leadership deliver results, even in the most difficult conditions.