Real-time annulus pressure while drilling for formation integrity test

Aspects of the disclosure can relate to a process for transmitting a pump-off pressure profile for formation integrity testing within a limited bandwidth. For example, a process may include measuring pump-off pressure data. The pump-off pressure data represents the pump-off pressure profile. The method also includes determining, from the pump-off pressure data, a pump-off pressure data portion corresponding to a formation integrity testing characteristic. The method also includes compressing pump-off pressure data portion with a compression protocol to produce compression bits. The compression bits representing the pump-off pressure data portion corresponding to the formation integrity testing characteristic. The method also includes transmitting, via a communication module, the compression bits to a computing device.

FIELD OF THE INVENTION

Aspects of the disclosure relate to oil and gas drilling and the recovery of hydrocarbons. More specifically, aspects relate to real-time annulus pressure which drilling for formation integrity tests.

BACKGROUND

Oil wells are created by drilling a hole into the earth using a drilling rig that rotates a drill string (e.g., drill pipe) having a drill bit attached thereto. The drill bit, aided by the weight of pipes (e.g., drill collars), cuts into rock within the earth to create a wellbore. Drilling fluid (e.g., mud) is pumped into the drill pipe and exits at the drill bit. The drilling fluid may be used to cool the bit, lift rock cuttings to the surface, at least partially prevent destabilization of the rock in the wellbore, and/or at least partially overcome the pressure of fluids inside the rock so that the fluids do not enter the wellbore. During the creation of the wellbore, these drilling rigs can measure the physical properties of the well environment. Data representing the measurements can be transmitted to the surface as pressure pulses in a mud system (e.g., mud pulse telemetry) of the oil well.

SUMMARY

Aspects of the disclosure can relate to a process for transmitting a pump-off pressure profile for formation integrity testing within a limited bandwidth. For example, a process may include measuring pump-off pressure data. The pump-off pressure data represents the pump-off pressure profile. The method also includes determining, from the pump-off pressure data, a pump-off pressure data portion corresponding to a formation integrity testing characteristic. The method also includes compressing pump-off pressure data portion with a compression protocol to produce compression bits. The compression bits representing the pump-off pressure data portion corresponding to the formation integrity testing characteristic. The method also includes transmitting, via a communication module, the compression bits to a computing device.

DETAILED DESCRIPTION

Embodiments described herein generally relate to methods for measuring pressure within a wellbore. More particularly, embodiments described herein relate to a system and a method for transmitting a pump-off pressure profile in a wellbore.

Formation integrity tests (FITs) can be performed during pumps-off to determine the maximum annular pressure that can be applied to the formation. Many measurements regarding formation properties (e.g., leak-off pressure, formation fracture pressure, minimum horizontal stress (σh), formation permeability, etc.) can be evaluated from a pumps-off annular pressure profile. The synthetic “downhole” pressure curve, however, may not represent the data properly in most cases and, sometimes, can lead to highly-deviated results. Many factors, such as mud compressibility, gel strength, mud type, and borehole temperature, can contribute to the large discrepancy. Thus, the present disclosure discloses transmitting an annular pressure profile in real-time (RT) for an accurate FIT analysis.

A large amount of annular pressure data may be accumulated during FITs. For example, a 1-hour FIT generates 1,800 pressure points at a sampling period of 2 seconds. Each pressure point utilizes 15 bits to cover the range of 0 psi to 30,000 psi with 1-psi resolution. The total transmission would take 37.5 minutes at a rate of 12 bits/second. Such delays can impact the normal drilling operations since the data from other measurements cannot be sent in a long time period after pumping resumes.

Thus, pressure data representing major turning points along the pressure curve can be transmitted uphole to provide suitable compression ratios and reduce real-time transmission time. The pressure data representing the major turning points allow tracing of the pressure changes and identifying some key measurements, such as leak-off pressure, formation fracture pressure, and so on. In some embodiments, however, where high-density pressure points are utilized for a detailed FIT analysis, the sparse samples may not provide enough information. An example is shown inFIG. 1. From the pressure curve from real-time pumps-off annulus pressure while drilling, the major changes along the FIT can be represented. But for the differential pressure (see the bottom ofFIG. 1), the real-time pumps-off annulus pressure while drilling pressure curve shows steps in long time intervals and the subtle changes may be missing.

The present disclosure discloses transmitting a portion of the FIT pressure curve profile. Thus, the present disclosure can be independent of real-time pumps-off annulus pressure while drilling due to different field usages. For example, the present disclosure transmits the FIT section from the start of FIT to the maximum pressure point (Pmax), as indicated inFIG. 1. Extensions on both sides are allowed and an operator can configure their time lengths according to the job requirements. The present disclosure detects the FIT section automatically and then performs compression. The compression bits are encapsulated into a sequence of multiple on-demand frames (MODFs) for real-time transmission. Respective MODFs can be decompressed independently into a part of pressure curve as this can be useful to reduce telemetry noise because part of FIT APWD data is available from the correctly-received MODFs. If a significant portion of the data is missing or corrupted, an operator can downlink to a logging while drilling (LWD) tool and request retransmission of the real-time FIT annulus pressure while drilling pressure curve.

During FIT, the annular pressure data is acquired by the pressure board in the LWD tool. When the pumps-up is detected (e.g., LTB power on), the present disclosure searches the pumps-off pressure profile and identifies (e.g., determines) the FIT section from the start of FIT test to Pmax(as shown the curve inFIGS. 2A-C). According to the job requirements, the operator is allowed to configure extensions on both sides to allow additional pressure data to be included to help the FIT analysis.

To achieve a high compression efficiency, the pressure data is filtered to remove the measurement noise and then decimated to a preselected sampling period. For example, the preselected sampling period may be a 4-second sampling period. The sampling period, however, can be modified according to the requirements of the data density. Thus, other sampling periods may be utilized. Additionally, differential coding can be applied for data compression because due to the continuity along the FIT annulus pressure while drilling curve.

The compression bits are then encapsulated into a sequence of MODFs for real-time (or near real-time) transmission. Independent MODFs can represent a portion of the FIT annulus pressure while drilling (APWD) curve independently. Thus, if one or more MODFs are lost or corrupted, other parts of FIT curve can still be reconstructed correctly from the available MODF(s).

When the MODFs are ready, the LWD transmits the MODFs to a measuring while drilling (MWD) via a suitable transmission protocol (e.g., Opcode 97), and the MWD arranges the MODFs to send the MODFs up-hole through mud-pulse telemetry.

At the surface, a computing device decodes the MODFs and then decompresses each MODF into part of the FIT curve and causes display of the real-time FIT annular while drilling curve in its time-domain log or in a cross-plot of pressure-vs-time. The computing device can also provide a FIT analysis.

The present disclosure first discusses detection of pump-off pressure data corresponding to a formation integrity test (e.g., pump-off pressure data having a formation integrity test characteristic) and compression of the pump-off pressure data having a formation integrity test characteristic. The present disclosure then discusses transmission protocols for transmitting the compressed pump-off pressure data uphole.

Compression Protocol

A compression protocol is now described for transmitting pressure data in real-time (or near real-time) representing the FIT annular pressure while drilling pressure curve utilizing mud-pulse telemetry. As described below, the compression protocol can comprise: 1) detection of the FIT section; 2) filtering and decimation of the pressure data; 3) quantization of the pressure data; and 4) differential coding of the pressure data.

Detection of FIT Section

The FIT section can be defined from the start of pressure build-up in FIT to Pmaxin the pumps-off pressure profile. The start of pressure build-up for a FIT is defined as a turning point where a substantial pressure change occurs. For example, the turning point can be assumed between the minimum pressure before Pmax(denoted as Pmax) and Pmax, as shown inFIG. 3.

To accelerate the detection process, in an example embodiment, the pressure points can be downsampled at 20-second sampling periods. To avoid the impact from measurement noise, a moving averaging can be taken during downsampling (e.g., the average value of respective pressure data in each 20-second window is assigned to the center point in that window).

The searching range is set from Pminto a middle pressure point at (Pmin+Pmax)/2, as shown as the red circles inFIG. 3. In cases where multiple middle points are available, the last middle point is selected as the end of searching range.

Then, let Pd(i), i=0,1, . . . ,Nddenote the Ndpressure points after downsampling. An initial threshold of pressure may be set as Pth=5 psi by the compression protocol and may be defined at the start of the FIT curve as:1. Compute the pressure difference ΔPd(i)=Pd(i+1)−Pd(i), i=0,1, . . . ,Nd−2.2. Find the latest index i* where ΔPd(i*)<Pth.3. If ΔPd(i)≥Pth, set i*=0 as the start of FIT and stop.4. Else if ΔPd(i)<Pth(i.e., i*=Nd−2), reduce the pressure threshold by half (i.e., Pth=Pth/2) and go to Step 2.5. Else, set i* as the start of FIT and stop.

The compression protocol allows extension data to be included on both sides of the FIT section in order to include more data according to the job requirements.FIG. 4shows an example of FIT annular pressure while drilling curve with 1-minute extension before the start of FIT and a 5-minute extension after Pmax.

Filtering and Decimation

The annular pressure while drilling data may contain measurement noise that can lower the compression efficiency. Moreover, the high sampling rate of original data might not be required for FIT analysis. Thus, in one or more embodiments, the pressure data having the FIT characteristics is filtered and decimated as described herein.

The frequency analysis directed to the FIT annular pressure while drilling data sets is shown inFIG. 5. A 10th order low-pass FIR filter may be applied to the pressure data at the cut-off frequency of 1/16 Hz and with the following coefficients:

The low-pass filter can also serve as an anti-aliasing filter for downsampling purposes.

Quantization

The filtered pressure data can be quantized as discussed herein. For example, let Pf(i), i=0,1, . . . ,Nfdenote the Nfpressure points after filtering and decimation. The quantization values can be calculated as q(i)=round(Pf(i)/Presolution), i=0, 1, . . . ,Nf, where Presolutionis the pre-defined quantization stepsize or resolution. In an example embodiment, Presolutionis set as 1 psi in order to favor an accurate FIT analysis and the quantization error is +/−0.5 psi.

Differential Coding and Adaptive Entropy Code Design

As shown inFIGS. 6A and 6B, different statistics histograms can be observed from various data sets. Each type of histogram may utilize a specific entropy coding for compression performance.

As such, an adaptive entropy code design that is adaptable to various data statistics is described. The adaptive entropy code design can be applied to the quantized pressure data. The adaptive entropy code design evaluates the histogram of difference values and generates a set of entropy codebooks from suitable Huffman code models. In an embodiment, there may be nine (9) Huffman code models utilized. The codebook using the least bits is selected to encode the data. The index of the optimal codebook in the set and the parameters to generate the codebook set are encoded and transmitted. During decompression, the codebook set is re-generated given the received parameters and the index is used to retrieve the codebook for decoding the difference values.

FIG. 7illustrates the nine (9) Huffman code models used to design the entropy codebook set. An example of coding six (6) items based on Huffman model (a) is shown inFIG. 8. The Huffman model first expands the model with binary splitting and generates enough branches for available items. Then, the coding bits for each item are constructed by outputting bit 0 or 1 on respective branches from left to right. Note that the binary splitting is performed downward and thus an increasing code length is expected from bottom to top.

Histogram Evaluation

The statistics histogram of difference values can be different from case to case. In one or more embodiments, an efficient entropy code design utilizes the structure of statistics histogram of quantized differential values. The statistics histograms vary from case to case. For simplicity purposes, three parameters are used in the present disclosure:

D0: The difference value of the highest occurrence. In case that multiple values have the same highest occurrence, the lowest one is selected as D0.

Dmaxand Dmin: The maximum and minimum difference value, respectively.

These three parameters are encoded and transmitted to describe the basic structure of a histogram. Let D*maxand D*mindenote the maximum value of Dmaxand the minimum value of Dmin, respectively. In this work, they are set as D*max=512 and D*min=−512. Then, Dmaxis encoded by applying the Huffman code model (a) on possible values of 0, 1, −1, 2, −2, 3, −3, . . . , D*max, D*min;

Dminis encoded by applying the Huffman code model (a) on possible values of Dmax, Dmax−1, . . . , D*min;

D0is encoded by applying the Huffman code model (a) on possible values of Dmin, Dmin+1, . . . , Dmax.

Codebook Set Design

The next step is to design entropy codebooks for respective difference values from Dminto Dmax. A lower number of occurrences is assumed for a value farther away from D0. The difference values are sorted in a decreasing order of occurrence based on their distance to D0. Then, an entropy codebook is designed by assigning less bits to values nearer to D0and more bits for ones farther from D0. A set of entropy codebooks are available from various sorting results and Huffman code models.

Sorting of Difference Values

In an embodiment, the sorting of difference values is performed as follows:1. Starting from D0, output a certain number of difference values as the 1stgroup.2. Output the remaining values on both sides of D0alternatively.3. To accelerate the ordering, the number of values to output on each side of D0is doubled after each output.
Definitions:

Wings: a wing is defined as values on the left or right side of D0, i.e., the left wing contains D−1, D−2, . . . , Dminand the right wing contains D1, D2, . . . , Dmax. A long wing is on the right if Dmax−D0≥D0−Dminand on the left if Dmax−D0<D0−Dmin.

N1stGroup: the number of the 1stgroup of difference values (starting from D0) to output for sorting.

NLongWingInThisStepand NShortWingInThisStep: the number of difference values to output in one step on the long and short wing, respectively.

In an example embodiment, the sorting process may comprise:

1. Output the 1stgroup of N1stGroupvalues by starting from D0on the long wing. If the size of long wing is lower than N1stGroup, (i.e., Dmaxor Dminis reached before N1stGroupvalues are output) continue to output the remaining values on the short wing in the direction away from D0.

2. Output NLongWingInThisStepor NShortWingInThisStepvalues by starting from next value to the last output one in this wing in the direction away from D0. If Dmaxor Dminis reached before the specific number of values are output, move to the opposite wing and output the remaining values in the direction away from D0.

3. Multiply NLongWingInThisStepor NShortWingInThisStepby 2 (i.e., double the number of values to output on this wing in the next step).

4. Move to the opposite wing if Dmaxor Dminhas not been reached on that wing. Otherwise, stay on the same wing as the last output value.

5. Repeat the steps from 2 by swinging between two wings until the difference values are output for sorting.

In an embodiment, an example with N1stGroup=2, NLongWingInThisStep=2 and NShortWingInThisStep=1 is given based on the histogram inFIG. 9:1. Output N1stGroup=2 values of Do and D1on the long (right) wing.2. Output NLongWingInThisStep=2 values of D2and D3on the long (right) wing3. Update NLongWingInThisStep=4;4. Move to the short (left) wing and output NShortWingInThisStep=1 value of D−1.5. Update NShortWingInThisStep=2;6. Move to the long (right) wing and suppose to output NLongWingInThisStep=4 values in this step. However, one value may within that wing. After outputting Dmax, move to the short (left) wing and continue to output Dmin.7. Stop since values have been output for sorting.

For respective Huffman code models, one codebook can be designed for the sorted difference values. In an example embodiment, the difference values are assumed to be sorted in the order of decreasing occurrence, so more bits are assigned to values which are output later, and the values which are output in the same step share the same “header” bits from the Huffman code model. Then Huffman code model (a) is used to code the “index” for each value if two or more values are in that group.

The code design for the above example based on Huffman code model (d) is illustrated inFIG. 10.

Given D0, Dmin, and Dmax, a set of codebooks are available with various Huffman code models and parameters of (N1stGroup, NLongWingInThisStep, NShortWingInThisStep). In an example, the maximum numbers are set as 6, 2 and 1, respectively, and the codebook set is designed as follows:

Initialization: empty the codebook sets CodebookSet.For N1stGroup=1:6For NLongWingInThisStep=1:2For NShortWingInThisStep=1:1Sort the difference values as described aboveFor each one of 9 Huffman code models (inFIG. 7)Design a codebook for values as described aboveIf this new codebook is NOT in CodebookSet (i.e., the code length for each value is different from previous codebooks), add this new codebook to CodebookSet.
Optimal Codebook Selection

In the codebook set (e.g., CodebookSet), the optimal entropy codebook is selected as the entropy codebook that encodes the most difference values within the available bandwidth. In practice, a binary searching protocol can be applied to code as many values as possible.

Let q(i), i=0,1, . . . ,Nq−1 denote Nqquantization values and Nbdenote the available number of bits. Then, he binary searching protocol can comprise:1. Encode the reference quantization value q(0) using Nrefbits. In an embodiment, Nrefcan be set to 15 bits to cover a pressure range of 0 psi to 30,000 psi with a quantization stepsize of Presolution=1 psi.2. Compute the difference values Δq(i)=q(i+1)−q(i), i=0,1, . . . ,Nq−2.3. Set i0=0, =i2=Nq−2.4. Evaluate the histogram of Δq(i), i=0,1, . . . ,i1and compute D0, Dminand Dmax.5. Encode Dmax, Dminand Do and generate Nhistcoding bits in total.6. Design the entropy codebook set.7. For each codebook in the set, evaluate the number of bits to code respective values Δq(i), i=0,1, . . . ,i1including the bits to index this codebook in the set (the index is encoded based on Huffman code model (a) as illustrated inFIG. 8).8. From the results in Step 7, determine the optimal codebook Codebook that generates the minimum number of bits N*diff.9. If (Nref+Nhist+N*diff)>Nb,a. If i1>i0+1, update i2=i1, i1=floor((i0+i2)/2) and go to Step 4.b. Else, set i*=i0as the index of last difference value to be coded.10. Else,a. If i1<i2−1, update i0=i1, i1=floor((i0+i2)/2) and go to Step 4.b. Else, set i*=i1as the index of last difference value to be coded.

Compression Bit Layout

The compression bitstream corresponding to the pressure data (e.g., the pressure data corresponding to the FIT section) can be constructed by concatenating coding bits as output (as described above). In some instances, different statistics histogram are expected before and after Pmax, two independent entropy coding schemes can be applied when Pmaxis included within the available bandwidth. In this case, the pressure points before and at Pmaxare first compressed into the minimum number of bits. Then, the remaining bits are used to code respective difference values after Pmax. As illustrated inFIGS. 11A-11D, the position of Pmaxis utilized to separate the two bitstreams. In this work, 8 bits are used to index up-to 256 points away from the reference pressure (i.e., a value 0 indicates Pmaxis the 1stone after the reference point). For the portion after Pmax, the coding of the reference pressure may not be included.

Once compressed, the compression bits are encapsulated into a sequence of MODFs for real-time transmission. In an embodiment, the MODFs can employ error protection to reduce bit errors from noisy mud-pulse telemetry. For example, the MODFs can employ a product single-parity check (PSPC) code error protection in accordance with an embodiment of the present disclosure.

Respective MODFs can be decompressed independently to reconstruct respective portions of the pressure curve. In the event that MODFs are missing or corrupted, the remaining MODFs can be used to reconstruct part of pressure curve, which may be sufficient for a FIT analysis.

The extension data after Pmaxis configurable in order to include additional information after the FIT section. In an embodiment, if extension data has been utilized and there is still space in the MODF, additional extension data after Pmaxcan included until the remaining bits are used up.

A single MODF definition can be utilized in accordance with the present disclosure, as shown inFIG. 12. For instance, the MODF can constructed with a frame synchronization word (24 bits for QPSK modulation), a 12-bit SubMODFID dpoint (4 bits are used to identify up-to 16 MODF definitions shared by the tools in the bottom hole assembly and the remaining 8 bits are parity bits for error protection), a 5-bit sequence number dpoint, a 12-bit timestamp dpoint, and 42 8-bit compression dpoints. The 42 compression dpoints can contain 300 information bits and 36 parity bits from a 17×18 PSPC error correcting code. These 300 information bits represent the available bandwidth in each MODF and are used to code the pressure points as described above.

The 5-bit sequence number dpoint is used to identify up-to 31 MODFs in a sequence. Its full-scaled value 31 is to indicate an LTB error during tool communication. This dpoint can be used to monitor the MODF sequence in field and help calculate the remaining time to compete the sequence. It also provides the possibility to re-call a specific missing or corrupted MODF by downlinking with its sequence number. The 12-bit timestamp dpoint is used to compute the time span from the acquisition of reference pressure to the time when MWD requests this dpoint.

Transmission of Multiple On-Demand Frame

The MODF sequence is transmitted following the survey and utility frames after the telemetry resumes, as illustrated inFIG. 13.

The transmission of the first MODF can be delayed if compression cannot be finished in time or MWD fails to obtain the MODF correctly upon the completion of utility frame. In an embodiment, repeating MODF frames are transmitted instead to avoid telemetry interference. Additionally, dpoints from other measurements (e.g., tool face) can be embedded in MODF to meet possible requirements on update rates. To avoid the gaps in real-time logs during the MODF transmission, the MODFs and repeating frames can be transmitted in an interleaving manner and mud pulse system dpoints (single-curve compression) in repeating frames can be used to fill up-to 200-second gaps.

The time interval between MODFs is programmable in order to adapt with various telemetry speeds. The time interval may be set to zero so that the whole MODF sequence is transmitted before any repeating frame and the latency of real-time FIT annular pressure while drilling pressure curve is minimized.

Experimental Results

During simulation, 1-minute and 5-minute extensions are assumed before the start of FIT and after Pmax, respectively. However it is understood that the extension after Pmaxcan be longer than 5 minutes in order to fill (e.g., complete) the last MODF.

FIG. 14illustrates a FIT annulus pressure while drilling curve, where the annulus pressure is increased at an almost fixed rate until it reaches the leak-off pressure. The blue curve is the recorded-mode (RM) data from a tool dump file and the red curve represents the real-time curve. From the differential pressure curve as shown at the bottom ofFIG. 14, the change at around 4,800 seconds indicates the presence of the leak-off pressure. In this embodiment, the total time length of real-time FIT annulus pressure while drilling pressure curve data is 51 minutes. This pressure cure utilizes five (5) MODFs, and transmission uphole takes 5.4 minutes at a 6 bits/second telemetry speed.

FIG. 15illustrates a case of a varying pressure increasing rate during FIT. As shown, the pressure changes are captured in the differential pressure curve. In this embodiment, the 23.9-minute FIT annulus pressure while drilling data is transmitted utilizing three (3) MODFs in a time of 3.2 minutes at a 6 bits/second telemetry speed.

A case of large pressure difference (up-to 30 psi after filtering) is shown inFIG. 16. As shown, two (2) MODFs are used to send the 9.4-minute FIT annulus pressure while drilling pressure curve at a transmission time or 2.2 minutes at a 6 bits/second telemetry speed.

FIG. 17shows an example of two FIT cycles. The FIT part with the highest Pmaxis selected for RT transmission. It uses 2 MODFs and takes 2.2 minutes to send the 13.3-minute FIT annulus pressure while drilling pressure profile curve at a 6 bits/second telemetry speed.