Successful operations in hostile well environments demand special instrumentation in combination with extensive expertise. Operators and service companies both face similar challenges - detailed conceptual planning and design must evolve to successfully log in high pressure, high pressure (HP/HT) environments.

Wireline logging in a high pressure/high temperature (HP/HT) environment requires special equipment as well as additional operational planning for the actual logging job. Operational excellence is essential to provide operators with the much needed wireline logging services in these hostile environments. HP/HT wells have the same logging and evaluation requirements as conventional wells; however, logging while drilling (LWD) is not available in hot wells, and consequently operators are dependent on quality wireline data.

Planning for tools and equipment

When planning for new tools and equipment, the primary question asked is, "what kind of data is needed?" If an oil company is drilling a well, priority is given to the single most important piece of data needed. If that piece of data is a deliverable, then other lower priority services may be addressed. Ideally, a system is designed to offer the greatest range of data.

Design phase

The design of hostile wireline logging equipment is challenging. HP/HT engineering involves a small corps of highly experienced specialists who usually have a past HP/HT electronics experience history. The normal break-over operating point for HP/HT specs is beyond 350?. Normal wireline logging tools are rated to 350?F (175?C). To put things in perspective, the stringent spec criteria demanded for HP/HT logging systems is somewhere between the specs of the aerospace and geothermal industries. There are few industry electronics experts that possess this level of design experience.

Suppliers have done a good job of upgrading materials used in logging systems, including seals, adhesives, rubber components, etc. For example, aerospace supplier Honeywell is very involved in upgrading electronics, both for their own uses and for other offshoot applications. Often it is difficult to distinguish the difference between standard logging tools and HP/HT tools externally unless they are compared side by side. Figure 1 illustrates the difference in the electronics module housing thickness of a normal XMAC acoustic tool compared to a hostile environment HP/HT XMAC.

Field testing

Since HP/HT services require a highly trained workforce to prepare specialized tools as well as maintain them in between jobs, centers of excellence have been established by the major service suppliers.
During the field test phase of new instrumentation, these centers of excellence assure that equipment is maintained consistently and feedback is given to the design engineers for any design modifications to the instrument. By keeping operational variables constant, valuable information and data relating to the development of the tool can be gained. Since HP/HT wells available for field testing may be limited, well conditions can be closely emulated in pressure/temperature pits to provide additional testing opportunities.
Operations

Pre-job planning and preparation. Early planning is one of the luxuries all HP/HT wells enjoy.
A service company's capabilities to provide HP/HT products is critical in an oil company's selection process. Getting involved early in the planning stage offers both parties the opportunity to address each other's requirements. Conveyance methods to match available instrumentation may also be discussed. Changes in trajectory may be recommended to accommodate wireline tools, or tool modifications may even be warranted if the need is great and if time permits.

Personnel experience and centers of excellence. Certain spots in the world are designated as HT or HP/HT areas, and special HP/HT or HT Districts near these areas are usually designated as "centers of excellence." Examples are the Baker Atlas district in Alice, Texas, a designated land district, while Houma, La., is a designated offshore center of excellence.

Maintaining "centers of excellence" plays a big part in allowing personnel to obtain invaluable amounts of specialized experience. Most tools are rated to their respective temperatures for a limited amount of time. Logging HP/HT wells is time-based, and the more experienced the crew, the quicker they can log a well and get their tools out of harm's way before the time exposed to hostile environments takes its toll. Experienced people in HP/HT logging can make critical decisions on the fly, often saving a logging run and eliminating unnecessary logging time.

Conveyance. Conveyance methods and strategy is a key element of hostile instrument performance. Logging via wireline as opposed to pipe-conveyed logging (PCL) is an operational decision to be considered. Conveyance modeling software can be run to determine if the wellbore's path will allow wireline conveyance or if an alternative method needs to be considered. Modeling is important for answering logging conveyance questions. At the onset of a logging job, the logging suite and the predicted wireline tension profiles must be determined. Cerberus for Wireline, a modeling software developed by CTES LC to predict the downhole behavior of wireline and slickline, provides answers with a high level of confidence for very complex situations.

Logging techniques

Time in the well and time the instrument is powered up are key factors in the capability of the instrumentation to perform in hostile equipment. Powering the tool up for a minimum amount of time limits the contributions of internal heating. Instrument specifications are normally considered for a specific time period. However, the performance of the instrument will vary dependent on the maximum pressure, temperature actually reached and the time deployed in the well (both powered and un-powered). Instrument ratings (temperature and pressure) are derived from proven design elements such as housing thickness, flasking of electronic components, pressure compensation and time powered up. In addition, key components have been heat- and pressure-tested.

Zero-Offset VSP case history

An oil company needed a zero-offset Vertical Seismic Profile (VSP) in an extremely hostile, deep gas well in East Texas with a well depth of 21,400 ft (6,523 m). The well bottomhole temperature (BHT) was believed to be 440?F (227?C), and the total depth (TD) hydrostatic pressure was estimated at 22,500 psi. Obtaining the actual value was critical to the operator, and it requested a surface - readout Temperature and Pressure combination. A production logging toolstring was deployed on a dedicated HP/HT logging unit. The actual BHT was determined to be 424?F (217?C), while the bottomhole pressure (BHP) was recorded at 15,450 psi. The information allowed the operator to run tubing conveyed perforating (TCP) guns and completion equipment utilizing the same logging unit. Several more trips were made to acquire the much-needed VSP data.

HT perforating case history

Ultra-hot wells are common in South Texas, where the geothermal gradient can be as high 2.3?F/100 ft (-16? C/30.5 m) in some the lower Frio formations that start at approximately 12,000 ft (3,660 m). Examples illustrating the success extensive pre-job planning and personnel experience produces were recently demonstrated in Edinburg, Texas. Utilizing their hot hole experience, the service company's field crew was able to meet the challenge of successfully perforating several ultra-hot wells reaching temperatures of 478?F (248?C) at depths up to 17,888 ft (5,452 m). In one particular well, a GR/CCL/TEMP log was first run to the depth of 17,888 ft (5,455 m) at 478?F (247? C) to position the guns. The first zone was perforated at 17,762 ft (5,414 m) with a bottom-hole temperature of 474?F (246?C) and 15,100 psi (1,041 bar). After perforating the first well, 9,400 psi (648 bar) was detected at the surface. On another well, completions were run at 455?F (235?C) with a total of 1,295 holes perforated. In addition to these perforating runs, the Edinburg crews ran gamma ray and cement bond logs at temperatures of 440?F (227?C) and 435?F (224?C).

Data quality control

Being able to mechanically log HP/HT wells is only part of the equation. Obtaining good-quality data and verifying that the data is valid is obviously essential. Often, HP/HT wells are drilled in rank wildcat areas where log responses can not be compared to nearby offset wells. Tools have to be characterized at temperature more rigorously than standard tools to ensure the data is accurate.

The HP/HT logging cycle does not end when the logging job is finished. Post-job maintenance is critical to ensure success in future logging jobs. Maintenance on HP/HT tools is normally performed on a point system. Points are accumulated for everything from time sitting on the rack in the shop to how many trips are made to location. More points are assigned when a tool goes into a well, even more points when used in hot wells and still more in hot wells with HP.