The investment is expected to allow Aberdeen-based SPEX to accelerate growth in international markets and the development of new technologies.
The company is the fourth Scottish oil and gas services business backed by the Business Growth Fund since it opened for businesss in April 2011.
The investments reflect the fund's confidence in the prospects for a sector that is enjoying rapid growth amid strong activity levels in the North Sea.
SPEX and other companies backed by the growth fund, such as Petrotechnics, are drawing on expertise developed in the North Sea to win business in a range of overseas markets.
Founded in 2009, SPEX has developed a range of tools used for processes like severing and closing wells.
The company recently opened operations in the US, Mexico and Saudi Arabia.
Chief executive Jamie Oag said the support from the fund will allow the company to increase investment in people, tools and manufacturing equipment. SPEX has around 60 employees.
The company increased turnover to £13.9m in 2012, from £7.2m in the preceding year. It grew pre-tax profits to £2.5m from £1.05m.
Mike Sibson, investment director at Business Growth Fund, said: " SPEX has built an excellent position based on a unique set of skills, and has exceptionally strong engineering capabilities."
The investment is the twelfth in Scotland by the bank-backed fund, which takes minority stakes in firms. The fund last week invested £7m in MONO Consultants, a specialist in telecommunications networks.
Separately, Lloyds Banking Group' private equity arm said it would invest £250m in niche manufacturing sectors such as oil and gas over the next two years.
The LDC business completed four oil services deals last year including backing a management buy out of Angus Fire, which supplies fire-fighting products.