Methods and a system for well logging communication

Abstract

Methods and a system for communication between the well logging surface equipment and downhole tools. A well logging communication system comprised of a surface modem, one or more downhole modems for communication to the surface through one or more signal channels of the wireline. A system with zero, one or multiple downhole tool buses.

Description

BACKGROUND OF THE RELATED ART

The arrays of high definition sensors in downhole tools generate large amounts of data on the borehole. A communication system is utilized to send the real-time data from the downhole tools to the surface. There are two types of data involved: uplink and downlink data. To get the expected data from downhole tools, data is sent from the surface to downhole tools for control; this data from surface to downhole tools is downlink data. The data sent from downhole tools to surface is called uplink data. The rate demanded for downlink data is low in comparison to the high uplink data rate needed.

Running multiple tools together in one string saves rig time therefore high compatibility is demanded between the tools. There are many downhole tool vendors on the market, and each company produces different series of tools. In a conventional system, the downhole modem is built as a standalone tool; this modem is also called telemetry. It is the only telemetry in the downhole tool string. The downhole tools exchange data through the downhole tool bus. Different tool suppliers have different downhole tool bus which makes it very difficult to run tools from different vendors in one string.

Orthogonal Frequency-Division Multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. The orthogonality among the subcarriers and between the real and imagery signal in one subcarrier, allows for high spectral efficiency. Accurate clock synchronization is critical for orthogonality of the signal. Most of high speed telemetry systems use OFDM modulation operated in continuous mode in which the modem occupies the signal all the time. Burst OFDM modulation allows modems to share the signal channel(s).

SUMMARY OF THE INVENTION

The problems mentioned in the background can be solved with a well logging communication system with multiple compact, low cost, low power downhole modems capable of using burst OFDM modulation. The downhole tools with modem are able to communicate to the surface through the wireline and communicate to the tools without modem through the local downhole buses.

The surface and downhole modems are capable of using any modulation. In the preferred embodiment, the uplink uses burst OFDM modulation to increase the speed and the downlink uses Phase Pilot Shift (PSK) modulation to simplify the design.

The embodiment is expected to transport 3 Mega-bit per second over the 7000 meters 7-conductor wireline cable and 300 kilo-bit per second over the single conductor wireline cable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional well logging communication system

FIG. 2 shows an exemplary well logging communication system of the embodiment

FIG. 3A and 3B show hardware block diagram of the modem.

FIGS. 4A and 4B show OFDM modulation block diagram of the modem.

FIG. 5 shows the processing flow of the interrupt routine after completion of a downlink data frame transmission.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a conventional well logging communication system. The surface modem 101 communicates to the modem 103 of downhole telemetry 104 through the wireline cable 102. The wireline cable 102 provides the uplink and downlink signal channels, provides power to the downhole tools, and holds the weight of whole downhole tool string. The downhole telemetry 104 communicates to the surface through the wireline cable 102 and to other downhole tools 105, 106, 107 through the downhole tool bus 108.

FIG. 2 shows an exemplary well logging communication system of the embodiment. The exemplary system shows how the system works with multiply downhole modems M1203, M2205, M3206, and multiple downhole tool buses 204, 207.

The surface modem 201 communicates to the downhole tools with modem 208, 210, 211 through the wireline cable 202.

The downhole tool 208 with modem 203 communicates to the surface through the wireline cable 202 and to one downhole tool 209 through the downhole tool bus 204.

The downhole tool with modem 210 communicates to the surface through the wireline cable 202 with no other tool attached.

The downhole tool 211 with modem 206 communicates to the surface through the wireline cable 202 and to two downhole tools 212, 213 through the downhole tool bus 207.

FIG. 3A shows hardware block diagram of the downhole modem. The Amplifier 301 receives, amplifies, and filters the downlink signal. The Analog to Digital Converter (ADC) 302 converts the amplified downlink signal to a digital signal. The Field-Programmable Gate Array (FPGA) 303 decodes the downlink signal from ADC 302 and encodes and outputs the encoded uplink signal to the Digital to Analog Converter (DAC) 304. The DAC 304 converts the digital uplink signal to analog signal. The liner driver 305 is an amplifier that is able to output enough power to drive the signal channel of the wireline cable. The analog switch 306 selects the signal channel of the wireline cable for the uplink data. The number of signal channel is not limited to three and can be any number less or equal to maximum signal channel the wireline can provide. Micro Controller Unit (MCU) 307 controls all the data flow. The modem acquires and controls other circuits through Interface connector 308.

FIG. 3B shows hardware block diagram of the surface modem. The surface modem hardware comprises of three identical uplink signal receiver 321,322,323, a downlink signal transmitter 324, and a FPGA 325.

FIG. 4A shows OFDM encoder block diagram of the downhole modem. The Scrambler 401 is used for energy dispersal on the carrier to reduce the inter-carrier signal interference. This eliminates the dependence of a signal's power spectrum upon the actual transmitted data. Reed-Solomon(R-S) is preferred for error correction.

Constellation Map 403 coverts the bit stream from R-S encoder 402 to real and imaginary amplitudes of all useable tones with the exception of the pilot tone. The signal of the pilot tone is used for clock synchronization and correction. A mapping code is used to generate different mapping results. The one with the lowest power is selected for output and the mapping code of the selected result is transported to surface in pilot tone.

The Fast Fourier Transform (FFT) 404 coverts the spectrum signal from Constellation Map 403 from frequency domain to time domain.

The DA Interface 405 inserts cyclic prefix (CP) between the symbols, and outputs the digital signal to hardware.

FIG. 4B shows one OFDM decoder block diagram of the surface modem. There is one decoder for each uplink signal channel; the decoders share some of the components to save resources.

The ADC interface 413 includes an Analog to Digital Converter (ADC) driver 413, a converting period modifier (CPM) 411, and an automatic Gain Control (AGC) 412.

The ADC driver 410 outputs the signals to the ADC hardware to complete the signal conversion from analog to digital and acquires the digital signal from the hardware. The CPM 411 generates signal to adjust the converting period of the ADC driver 410 and the AGC driver 412 sets the gain of the receiver hardware.

The filter block 414 mainly consists of a band pass filter to get rid of the noise and a Time Domain Equalizer (TEQ) to minimize the Guard Interval (GI) length.

The GI Remover 415 detects the start of the signal frame, finds out boundary of each symbol, removes the GI, and passes the signal inside the data window to the Inverse Fast Fourier Transfer (IFFT) 416. The signal is then transformed from time domain to frequency domain in IFFT 416.

The received signal is attenuated and rotated through the signal channel; the Inverse Channel Transfer Function (ICFT) is applied to the signal in Frequency Domain Equalizer (FEQ)−1 417 to recover transmitted amplitude and phase of the signal.

The Micro Controller Unit (MCU) 418 calculates the Pilot Correction Angle (PCA) for Clock Correction Calculator (CCC) 419, the Converting Period Adjustment CPA for ADC interface 410, and the data window adjusting signal for GI Remover 415.

The CCC 419 calculates the cosine and sine of the correction angle for all subcarriers. The formula for the correction angle is: PCA * tone number of the subcarrier/tone number of the pilot tone.

The FEQ−2 420 rotates the symbol to predetermined position by using the cosine and sine values of all subcarriers to correct the clock synchronization error.

The Constellation Demap 421 uses the mapping code does the inverse processing of Constellation Map 403. The R-S Decoder 422 does the inverse processing of R-S Encoder 402 and the Descrambler 423 does the inverse processing of scrambler 401.

FIG. 5 shows the processing flow of the interrupt routine after completion of a downlink data frame transmission. All downhole modems are trained before send uplink data frame. The decoding parameters of all downhole modem from the training are saved in surface modem. After sends a frame of downlink data, the surface modem reads the sync and destination of the downlink data frame; the modem does not set up any decoding channel if it's no response sync 502. The surface modem uses the destination as an index to find out the downhole modem to send response; the downhole modem is used to find the uplink signal channel which will be used to receive the response uplink signal 503. The surface modem loads the decoding parameter of the downhole modem to the decoder of the signal channel 504. The decoder of the signal channel is ready to decode the uplink signal at this point.

While the invention has been descripted in related to the use of wireline, the methodology of the invention can be applied to and data communication system with one or more signal channels.

Claims

1. A well logging communication system comprising of: a surface modem, one or more downhole tools with or without modem, minimum of one signal channel through the wireline cable between the surface modem and downhole modem(s), and some number of downhole tool buses between the downhole tools.

2. In the communication system of claim 1, wherein the surface modem comprises one downlink encoder, and one or more uplink decoders.

3. A method for well logging communication, comprising: deploying one surface modem, one or more downhole modems.

4. In method of claim 3, wherein the surface modem and downhole modems are able to deploy different modulations.

5. In method of claim 3, wherein the surface modem and downhole modem(s) time share the signal channel(s).

6. In method of claim 3, wherein the downlink signals contains the information for the surface modem to select the correct parameters for all modules of the decoder with the information including but not limited to: whether it requests uplink signal, and the downhole modem identity.

7. A method for running tools with multiple tool bus in one string, comprising: deploying at least one tool with embedded modem for each tool bus.

8. In method of claim 7, the tool with modem communicates to the surface via the wireline cable.

9. In method of claim 7, the tool without modem communicates to the surface through the tool with modem in the same tool bus.