The evaluation of fluid flows within a well bore is a frequently encountered problem in the oil and gas production industry. There are a number of different flow regimes including multi-phase fluid flows. Factors influencing the flow regimes can include a degree of borehole deviation and proportion of the phases, relative differences in phase densities, surface tension and viscosity of the phases as well as velocity, pressure and temperature. Understanding the fluid flow regime in a well may be used to understand the performance of a production well. A production log records one or more in-situ measurements that describe the nature and behavior of fluids in or around the borehole during a production operation, including an injection operation. Production logs can provide for example information about dynamic well performance and the productivity or injectivity of different zones. This information may be used to help diagnose problem wells or monitor the results of a stimulation or completion.

Characterization of a fluid flow especially in horizontal wells is very uncertain. Normally, flow rates are measured by means of a conventional spinner, and fluid velocity is highly dependent on conventional spinner survey. Unfortunately, spinners have a lot of limitations especially in heavy viscous fluids, low rate environments and turbulent flow.

It is an object of the invention to provide a downhole device that is designed to obtain reliable data on the fluid flow within a wellbore. The device must be able to adapt to the pressure and temperature regime within the well and be able to obtain data when the well is flowing or shut.

The object is met with a device as addressed above, which is equipped with a pressure compensation system comprising a spring means supported by a spring holder, a piston in contact with the spring means, a pressure compensation fluid chamber and seals sealing the pressure compensation chamber against the piston.

Downhole viscosity determination is difficult to accomplish and has so far not been used routinely. On the other hand, viscosity is a parameter that gives information on the constitution of downhole fluids, fluid flows and the quality of crude oil in a well. One method to determine the viscosity of a medium is by torque measurement.

Accordingly, the present device comprises a motor and a spinner, the spinner being arranged in a spinner head at the front end of the device, and the motor being coupled to the spinner. A viscosity sensoring device measures the energy consumption of the spinner within the surrounding medium, which depends on the viscosity of the medium and allows, by calibration, the determination of the nature of the surrounding medium, e.g. gas, water, oil or grease, and even the quality of the oil. Thus, the torque measurement yields information important for the evaluation of an oil well. The device of the invention comprises at least one sensor each for measuring relevant data within the well. Relevant data are in particular data on the fluid viscosity, pressure and temperature. The sensors are preferably arranged in the form of a tool string. Besides the flow data obtained by the spinner, most important are the pressure and viscosity data. The measuring module is equipped with at least one viscosity sensor and preferably more than one pressure sensor in order to register pressure differences. Two pressure and two temperature sensors are preferred.

For differential pressure measurement, the sensors are mounted at a distance. Preferably, one pressure sensor is in or close to the spinner head and the other one at or close to the tail of the device. The same holds for the temperature sensors. In order to obtain reliable data, it is important to have a pressure compensated system. Only with this compensation, the sensors and electronic elements will provide data that allow the precise prediction of a production rate, besides the data from the spinner.

The pressure compensation system is needed when the inside and the outside pressure are different along the downhole tool. The pressure compensation system is designed to create the pressure balance between the inside and outside of the tool in order to avoid a pressure difference that may cause leakage.

When positioning the tool downhole, the ambient temperature will increase dramatically. This will also increase the temperature of the pressure compensation fluid inside the chamber. The pressure compensation fluid normally is an oil, e.g. a combustion oil. The volume of the fluid will expand with the temperature increase. The expanded oil will exert a pressure on to the piston within the pressure compensation system. When the oil pressure within the system is higher than the outside pressure and the pressure supplied to the piston is big enough to compress the spring, the oil moves the piston against the spring. When the oil is fully expanded, the piston will stop moving, thus acting as a pressure release valve. At this time, the pressure inside the pressure compensation chamber will equal the outside pressure. Pressure balance is reached. The pressure compensation system provides a means to compensate for pressure and temperature changes within and outside the device. Predominantly the pressure compensation system is needed when moving the device downhole to the location of its operation and when tripping out the tool from the well. The system provides for constant and reproducible conditions within the device. This allows the system to produce reliable data with its sensors.

The downhole device of the invention generally consists of a tube-like housing, which is more or less conventional, a power supply, a motor coupled to a conventional spinner, at least one sensor, and the pressure compensation system. The housing is an elongate tube having the spinner at its head and a plug at its tail and the working elements inside. There is a power supply, which normally is a battery, but can also be an electrical cable reaching downhole. The battery provides power to a motor for driving the spinner in the head section. At least part of the sensors are arranged in the head section. Preferably, the device is equipped with phased pressure and temperature sensors.

The pressure compensation system comprises a spring means, preferably a compression spring, which is supported by a spring holder. In addition, there may be a spring guide attached to the spring holder which guides the longitudinal extension and compression of the spring. The spring, at it's other end, faces a piston, which is movably arranged within a ceramic sleeve. The piston comprises a sealing system made up by O-rings and for U-cup rings providing a seal between the distant body and the sleeve. The sealing rings seal the adjacent pressure compensation fluid chamber against the piston. The chamber itself is also sealed to the head section of the device by means of sealing rings, preferably O-rings.

For filling the chamber with the pressure compensation fluid, normally an oil, there is a filling port arranged at the wall of the housing.

The invention is further illustrated by the attached drawings. In the drawings Fig. 1 : shows a device of the invention; Fig. 2 is a sectional drawing of the device of

Fig. 1 ;

Fig. 3 shows the pressure compensation system of the device of Fig. 1 and 2;

Fig. 4 shows an enlarged drawing of the spring/piston section of Fig. 3; and

Fig. 5 a diagram showing exemplarily a torque diagram in various media.

Figure 1 is a drawing of the inventive device (1 ) with the housing (2), the spinner head (3) and the sealing plug (4) at the tail.

Figure 2 is a sectional view of the device of fig. 1 . The device is divided into two parts, the battery section B housing the battery (5) and the motor section M housing the motor control (6), the motor (7), the piston chamber (8) and the spinner head (3). A battery connector (BC) isolates the battery section from the motor section. In case of any leakage in the battery section the main module will not be effected. A motor connector MC forms the isolating part between the motor and the electronic elements. The motor connector separates the high pressure section (motor and spinner) from the electronics.

Figure 3 is a sectional view of the pressure control system (10) with a central driving shaft (9) and a compression spring (1 1 ) arranged around the driving shaft (9). The compression spring supports a piston (1 2) also enclosing the driving shaft (9) with sealing rings (1 3). Adjacent to the spring/piston combination ( /1 2) is a pressure compensation chamber PCC, which is sealed against the piston (12). Figure 4 gives details of the spring/piston arrangement of fig. 3. The spring (1 1 ) arranged around the driving shaft (9) is supported by the spring holder ( 6) which extends along the driving shaft (9) as a guide of the spring (1 1 ). The spring (1 1 ) ends at the piston (1 2) which itself is movable along the driving shaft (9) in a longitudinal direction of the device. Sealing rings (13) in form of U-cup seals provide sealing against a ceramic sleeve (18) and the driving shaft (9).

Adjacent to the piston is the pressure compensation chamber (15) which in operation is filled with an oil, e.g. a normal combustion oil. The chamber has a filling port (14), which also has a valve function. Sealing rings (17) provide tightness against housing elements.

A temperature rise in the pressure compensation chamber PCC results in an expansion of the oil, which drives the piston (12) in direction of the device tail. At the same time, compression spring (1 1 ) is compressed, until a pressure equilibrium is reached. On the other hand, in case of a temperature drop, e.g. when the device is retrieved from a well bore, the spring expands to compensate for the diminishing volume of the cooling oil in the chamber.

The ceramic sleeve (18) guiding the piston (12) is pre-bonded onto the piston housing. It provides less friction on the seal rings than the piston housing and is easy to be replaced in case of damage. The U-cup seals are selected because they have better sealing performance in the dynamic application comparing to regular O-rings.

The device of the invention is designed to help the interpretation of fluid flow and to give more reliable data on the flow rate. The pressure and viscosity data may be used according to the following calculations.

Calculations:

_ _ XMP

^V ~ yt. (1 )

In wellbore, ~ i ^WJ

^V ~ M (3)

Where,

Q: Rates Index, cm3 K: Permeability, D

A: Cross sectional area of wellbore or production casing, cm2

L: Length between sensors, cm

μ: Viscosity _j Qp

C: Constant

P: Pressure

Figure 5 shows in a diagram the torque index vs. time of a spinner in a surrounding medium. The torque index is given on a relative scale showing the variation of the medium viscosity in a well over the time, e.g. when downing the device into a well.

The low torque index of 10 to 15 in the first 2200 seconds indicates water. Thereafter, the device shows a zone of crude oil, as indicated by a torque index of 16 to 22. After about 9000 seconds a high torque index of about 70 to 80 indicates a viscous medium with a waxy or greasy consistence, before falling back to a low value below 10.

The torque measurement even a))ows the determination of the quality of crude oil, namely the distinction between light, intermediate and heavy qualities.

The data obtained by the viscosity and pressure measurements are regularly more precise than the data obtained by a regular spinner measurements.