Hydrocyclones have been used as a mode of sand separation within oil industry for many decades. The type generally used in oil and gas industry is the solid-liquid hydrocyclone and will be termed as “desander” throughout this manuscript. Desanders have been used all over the industry for sand separation in applications related to separation of mud from drilling fluids during drilling, during the initial testing of exploratory wells wherein sand production is anticipated, for post frac production during the clean-up and also during production phase of the wells with sand ingress. (Figure 1)
There are other methods used in the area of sand separation in surface facilities for the similar purpose like sand jetting inside vessels, using vessels itself as a means of separating solids via deposition and later cleaning them on regular basis and also conventional desander system. This paper would mainly deal in the latter conventional system.
When separating sand from a multi-phase or single phase stream, it is not only hydrocylone but is the installation of an entire system which can separate sand from the system, clean the separated sand and even includes the transfer of cleaned sand to safe area. Rawlins and Wang in their paper have detailed in simple terms the basic designing of solid handling systems constituting of five key areas (Figure-2) i.e. separation of solids (via hydrocyclone or jetting or filters), collection of solids (via an accumulator), cleaning of solids from oil contamination (via washing systems), dewatering (disposal of liquid slurry being separated from cleaned sand) and haulage (transportation of sand to onshore or disposal as per local authority requirement.
Further, depending on the location, desanders in a production platform can be classified into various categories on the locations it is installed:
- Well head desanders – installation of hydrocyclone system with accumulator and flushing mechanism preferably at the upstream of choke designed for the particular well condition and flow rates;
- Bulk desander – to cater to many well streams and located preferably at the downstream of header;
- Other Locations – these are specific locations within the facility like separation of sand downstream of separator vessels, or installation of a single phase hydrocyclone at upstream of crude oil transfer pump or cleaning up the well post completion or post fracturing.
Later sections will discuss the key factors important for selection of bulk or well head or other desander. Company had been facing serious sand production issues in many of their fields since last few decades. To resolve the issues related to sand, a dedicated team was set-up and a structured approach (Holistic Sand management approach) was applied since 2014. The approach constituted of development of a centralized multi-disciplinary team (as the company operates on an asset based model) named as ‘Central Sand Team’ (CST) along with asset based Regional Sand Teams. The approach till date has resulted in developing a culture within the Company to tackle sand in a focussed way. The approach includes consideration of all aspects of sand within the system right from sub-surface, surface handling and disposal. The work on desanders detailed in this paper is a part of the entire Holistic Sand Management approach.
With holistic sand management approach being applied in various fields and platforms, the initial study recommended installation of desanders in a few platforms to handle sand at surface for integrity of the facility and production optimization. This led to the need of looking into retrospect on the performance of existing desanders and strategize the selection, procurement and installation of upcoming desanders.
The entire enhancement of process to combat the challenges of >10 years was planned to be conducted in following way:
- Initial data collection of existing desanders;
- Finding gaps in technical selection and entire procurement strategy, if any;
- Conduct market survey to gauge technical capability / technology in the current market;
- Strategize on technical selection of desander;
- Strategize on procurement and contract management strategy.
During the study of past installations of desander systems within the Company, team had to dig up earlier records some of them were available and others not.
The team collected data from the existing desanders along with the results and performance. The main highlights of the initial findings i.e. company experience on past decade on the desanders (all well head type) were as follows:
- Total number of desander installations: 33
- Operational i.e. separating sand: 11
- Installations on-going till the results were out: 7
- Non-operational: 15
On the non-operational part, there were 7 desanders which were out of specifications i.e. the flow rate for which they were designed and installed were non-existence and due to all the 7 being in a category (mentioned above), no retrofit can be done. They were termed under absolute. 5 out of 15 had maintenance issues due to erosion in liners, etc. or no spare parts and rest had the well issues and therefore were nonoperational. Out of the 33, only 6 were under Category B and rest all were Category A. It is to be highlighted that even for the desanders which were termed as operational, it was characterized so, because the sand was being collected in the accumulator. Uncertainties still lied on the efficiency and the amount of sand being carry over downstream of desander.
It was concluded from the entire data collection and basic analysis that there was a gap in a standard process to select, procure, install and utilize desander systems. First objective like any other process improvement effort was to identify gaps.